diff --git a/core/lib/Drupal/Core/Test/Exception/MissingGroupException.php b/core/lib/Drupal/Core/Test/Exception/MissingGroupException.php
new file mode 100644
index 0000000000000000000000000000000000000000..448b4f5b2d75994c2a69cbee89d0ac05363fa6da
--- /dev/null
+++ b/core/lib/Drupal/Core/Test/Exception/MissingGroupException.php
@@ -0,0 +1,11 @@
+<?php
+
+namespace Drupal\Core\Test\Exception;
+
+/**
+ * Exception thrown when a test class is missing an @group annotation.
+ *
+ * @see \Drupal\Core\Test\TestDiscovery::getTestClasses()
+ */
+class MissingGroupException extends \LogicException {
+}
diff --git a/core/lib/Drupal/Core/Test/TestDiscovery.php b/core/lib/Drupal/Core/Test/TestDiscovery.php
new file mode 100644
index 0000000000000000000000000000000000000000..a8374f92a225c1746db95445fe9a22230712a289
--- /dev/null
+++ b/core/lib/Drupal/Core/Test/TestDiscovery.php
@@ -0,0 +1,487 @@
+<?php
+
+namespace Drupal\Core\Test;
+
+use Doctrine\Common\Reflection\StaticReflectionParser;
+use Drupal\Component\Annotation\Reflection\MockFileFinder;
+use Drupal\Component\Utility\NestedArray;
+use Drupal\Core\Extension\ExtensionDiscovery;
+use Drupal\Core\Test\Exception\MissingGroupException;
+use PHPUnit_Util_Test;
+
+/**
+ * Discovers available tests.
+ */
+class TestDiscovery {
+
+  /**
+   * The class loader.
+   *
+   * @var \Composer\Autoload\ClassLoader
+   */
+  protected $classLoader;
+
+  /**
+   * Statically cached list of test classes.
+   *
+   * @var array
+   */
+  protected $testClasses;
+
+  /**
+   * Cached map of all test namespaces to respective directories.
+   *
+   * @var array
+   */
+  protected $testNamespaces;
+
+  /**
+   * Cached list of all available extension names, keyed by extension type.
+   *
+   * @var array
+   */
+  protected $availableExtensions;
+
+  /**
+   * The app root.
+   *
+   * @var string
+   */
+  protected $root;
+
+  /**
+   * Constructs a new test discovery.
+   *
+   * @param string $root
+   *   The app root.
+   * @param $class_loader
+   *   The class loader. Normally Composer's ClassLoader, as included by the
+   *   front controller, but may also be decorated; e.g.,
+   *   \Symfony\Component\ClassLoader\ApcClassLoader.
+   */
+  public function __construct($root, $class_loader) {
+    $this->root = $root;
+    $this->classLoader = $class_loader;
+  }
+
+  /**
+   * Registers test namespaces of all extensions and core test classes.
+   *
+   * @return array
+   *   An associative array whose keys are PSR-4 namespace prefixes and whose
+   *   values are directory names.
+   */
+  public function registerTestNamespaces() {
+    if (isset($this->testNamespaces)) {
+      return $this->testNamespaces;
+    }
+    $this->testNamespaces = [];
+
+    $existing = $this->classLoader->getPrefixesPsr4();
+
+    // Add PHPUnit test namespaces of Drupal core.
+    $this->testNamespaces['Drupal\\Tests\\'] = [$this->root . '/core/tests/Drupal/Tests'];
+    $this->testNamespaces['Drupal\\KernelTests\\'] = [$this->root . '/core/tests/Drupal/KernelTests'];
+    $this->testNamespaces['Drupal\\FunctionalTests\\'] = [$this->root . '/core/tests/Drupal/FunctionalTests'];
+    $this->testNamespaces['Drupal\\FunctionalJavascriptTests\\'] = [$this->root . '/core/tests/Drupal/FunctionalJavascriptTests'];
+
+    $this->availableExtensions = [];
+    foreach ($this->getExtensions() as $name => $extension) {
+      $this->availableExtensions[$extension->getType()][$name] = $name;
+
+      $base_path = $this->root . '/' . $extension->getPath();
+
+      // Add namespace of disabled/uninstalled extensions.
+      if (!isset($existing["Drupal\\$name\\"])) {
+        $this->classLoader->addPsr4("Drupal\\$name\\", "$base_path/src");
+      }
+      // Add Simpletest test namespace.
+      $this->testNamespaces["Drupal\\$name\\Tests\\"][] = "$base_path/src/Tests";
+
+      // Add PHPUnit test namespaces.
+      $this->testNamespaces["Drupal\\Tests\\$name\\Unit\\"][] = "$base_path/tests/src/Unit";
+      $this->testNamespaces["Drupal\\Tests\\$name\\Kernel\\"][] = "$base_path/tests/src/Kernel";
+      $this->testNamespaces["Drupal\\Tests\\$name\\Functional\\"][] = "$base_path/tests/src/Functional";
+      $this->testNamespaces["Drupal\\Tests\\$name\\FunctionalJavascript\\"][] = "$base_path/tests/src/FunctionalJavascript";
+
+      // Add discovery for traits which are shared between different test
+      // suites.
+      $this->testNamespaces["Drupal\\Tests\\$name\\Traits\\"][] = "$base_path/tests/src/Traits";
+    }
+
+    foreach ($this->testNamespaces as $prefix => $paths) {
+      $this->classLoader->addPsr4($prefix, $paths);
+    }
+
+    return $this->testNamespaces;
+  }
+
+  /**
+   * Discovers all available tests in all extensions.
+   *
+   * @param string $extension
+   *   (optional) The name of an extension to limit discovery to; e.g., 'node'.
+   * @param string[] $types
+   *   An array of included test types.
+   *
+   * @return array
+   *   An array of tests keyed by the the group name. If a test is annotated to
+   *   belong to multiple groups, it will appear under all group keys it belongs
+   *   to.
+   * @code
+   *     $groups['block'] => array(
+   *       'Drupal\Tests\block\Functional\BlockTest' => array(
+   *         'name' => 'Drupal\Tests\block\Functional\BlockTest',
+   *         'description' => 'Tests block UI CRUD functionality.',
+   *         'group' => 'block',
+   *         'groups' => ['block', 'group2', 'group3'],
+   *       ),
+   *     );
+   * @endcode
+   *
+   * @todo Remove singular grouping; retain list of groups in 'group' key.
+   * @see https://www.drupal.org/node/2296615
+   */
+  public function getTestClasses($extension = NULL, array $types = []) {
+    if (!isset($extension) && empty($types)) {
+      if (!empty($this->testClasses)) {
+        return $this->testClasses;
+      }
+    }
+    $list = [];
+
+    $classmap = $this->findAllClassFiles($extension);
+
+    // Prevent expensive class loader lookups for each reflected test class by
+    // registering the complete classmap of test classes to the class loader.
+    // This also ensures that test classes are loaded from the discovered
+    // pathnames; a namespace/classname mismatch will throw an exception.
+    $this->classLoader->addClassMap($classmap);
+
+    foreach ($classmap as $classname => $pathname) {
+      $finder = MockFileFinder::create($pathname);
+      $parser = new StaticReflectionParser($classname, $finder, TRUE);
+      try {
+        $info = static::getTestInfo($classname, $parser->getDocComment());
+      }
+      catch (MissingGroupException $e) {
+        // If the class name ends in Test and is not a migrate table dump.
+        if (preg_match('/Test$/', $classname) && strpos($classname, 'migrate_drupal\Tests\Table') === FALSE) {
+          throw $e;
+        }
+        // If the class is @group annotation just skip it. Most likely it is an
+        // abstract class, trait or test fixture.
+        continue;
+      }
+      // Skip this test class if it is a Simpletest-based test and requires
+      // unavailable modules. TestDiscovery should not filter out module
+      // requirements for PHPUnit-based test classes.
+      // @todo Move this behavior to \Drupal\simpletest\TestBase so tests can be
+      //       marked as skipped, instead.
+      // @see https://www.drupal.org/node/1273478
+      if ($info['type'] == 'Simpletest') {
+        if (!empty($info['requires']['module'])) {
+          if (array_diff($info['requires']['module'], $this->availableExtensions['module'])) {
+            continue;
+          }
+        }
+      }
+
+      foreach ($info['groups'] as $group) {
+        $list[$group][$classname] = $info;
+      }
+    }
+
+    // Sort the groups and tests within the groups by name.
+    uksort($list, 'strnatcasecmp');
+    foreach ($list as &$tests) {
+      uksort($tests, 'strnatcasecmp');
+    }
+
+    if (!isset($extension) && empty($types)) {
+      $this->testClasses = $list;
+    }
+
+    if ($types) {
+      $list = NestedArray::filter($list, function ($element) use ($types) {
+        return !(is_array($element) && isset($element['type']) && !in_array($element['type'], $types));
+      });
+    }
+
+    return $list;
+  }
+
+  /**
+   * Discovers all class files in all available extensions.
+   *
+   * @param string $extension
+   *   (optional) The name of an extension to limit discovery to; e.g., 'node'.
+   *
+   * @return array
+   *   A classmap containing all discovered class files; i.e., a map of
+   *   fully-qualified classnames to pathnames.
+   */
+  public function findAllClassFiles($extension = NULL) {
+    $classmap = [];
+    $namespaces = $this->registerTestNamespaces();
+    if (isset($extension)) {
+      // Include tests in the \Drupal\Tests\{$extension} namespace.
+      $pattern = "/Drupal\\\(Tests\\\)?$extension\\\/";
+      $namespaces = array_intersect_key($namespaces, array_flip(preg_grep($pattern, array_keys($namespaces))));
+    }
+    foreach ($namespaces as $namespace => $paths) {
+      foreach ($paths as $path) {
+        if (!is_dir($path)) {
+          continue;
+        }
+        $classmap += static::scanDirectory($namespace, $path);
+      }
+    }
+    return $classmap;
+  }
+
+  /**
+   * Scans a given directory for class files.
+   *
+   * @param string $namespace_prefix
+   *   The namespace prefix to use for discovered classes. Must contain a
+   *   trailing namespace separator (backslash).
+   *   For example: 'Drupal\\node\\Tests\\'
+   * @param string $path
+   *   The directory path to scan.
+   *   For example: '/path/to/drupal/core/modules/node/tests/src'
+   *
+   * @return array
+   *   An associative array whose keys are fully-qualified class names and whose
+   *   values are corresponding filesystem pathnames.
+   *
+   * @throws \InvalidArgumentException
+   *   If $namespace_prefix does not end in a namespace separator (backslash).
+   *
+   * @todo Limit to '*Test.php' files (~10% less files to reflect/introspect).
+   * @see https://www.drupal.org/node/2296635
+   */
+  public static function scanDirectory($namespace_prefix, $path) {
+    if (substr($namespace_prefix, -1) !== '\\') {
+      throw new \InvalidArgumentException("Namespace prefix for $path must contain a trailing namespace separator.");
+    }
+    $flags = \FilesystemIterator::UNIX_PATHS;
+    $flags |= \FilesystemIterator::SKIP_DOTS;
+    $flags |= \FilesystemIterator::FOLLOW_SYMLINKS;
+    $flags |= \FilesystemIterator::CURRENT_AS_SELF;
+    $flags |= \FilesystemIterator::KEY_AS_FILENAME;
+
+    $iterator = new \RecursiveDirectoryIterator($path, $flags);
+    $filter = new \RecursiveCallbackFilterIterator($iterator, function ($current, $file_name, $iterator) {
+      if ($iterator->hasChildren()) {
+        return TRUE;
+      }
+      // We don't want to discover abstract TestBase classes, traits or
+      // interfaces. They can be deprecated and will call @trigger_error()
+      // during discovery.
+      return substr($file_name, -4) === '.php' &&
+        substr($file_name, -12) !== 'TestBase.php' &&
+        substr($file_name, -9) !== 'Trait.php' &&
+        substr($file_name, -13) !== 'Interface.php';
+    });
+    $files = new \RecursiveIteratorIterator($filter);
+    $classes = [];
+    foreach ($files as $fileinfo) {
+      $class = $namespace_prefix;
+      if ('' !== $subpath = $fileinfo->getSubPath()) {
+        $class .= strtr($subpath, '/', '\\') . '\\';
+      }
+      $class .= $fileinfo->getBasename('.php');
+      $classes[$class] = $fileinfo->getPathname();
+    }
+    return $classes;
+  }
+
+  /**
+   * Retrieves information about a test class for UI purposes.
+   *
+   * @param string $classname
+   *   The test classname.
+   * @param string $doc_comment
+   *   (optional) The class PHPDoc comment. If not passed in reflection will be
+   *   used but this is very expensive when parsing all the test classes.
+   *
+   * @return array
+   *   An associative array containing:
+   *   - name: The test class name.
+   *   - description: The test (PHPDoc) summary.
+   *   - group: The test's first @group (parsed from PHPDoc annotations).
+   *   - groups: All of the test's @group annotations, as an array (parsed from
+   *     PHPDoc annotations).
+   *   - requires: An associative array containing test requirements parsed from
+   *     PHPDoc annotations:
+   *     - module: List of Drupal module extension names the test depends on.
+   *
+   * @throws \Drupal\simpletest\Exception\MissingGroupException
+   *   If the class does not have a @group annotation.
+   */
+  public static function getTestInfo($classname, $doc_comment = NULL) {
+    if ($doc_comment === NULL) {
+      $reflection = new \ReflectionClass($classname);
+      $doc_comment = $reflection->getDocComment();
+    }
+    $info = [
+      'name' => $classname,
+    ];
+    $annotations = [];
+    // Look for annotations, allow an arbitrary amount of spaces before the
+    // * but nothing else.
+    preg_match_all('/^[ ]*\* \@([^\s]*) (.*$)/m', $doc_comment, $matches);
+    if (isset($matches[1])) {
+      foreach ($matches[1] as $key => $annotation) {
+        // For historical reasons, there is a single-value 'group' result key
+        // and a 'groups' key as an array.
+        if ($annotation === 'group') {
+          $annotations['groups'][] = $matches[2][$key];
+        }
+        if (!empty($annotations[$annotation])) {
+          // Only @group is allowed to have more than one annotation, in the
+          // 'groups' key. Other annotations only have one value per key.
+          continue;
+        }
+        $annotations[$annotation] = $matches[2][$key];
+      }
+    }
+
+    if (empty($annotations['group'])) {
+      // Concrete tests must have a group.
+      throw new MissingGroupException(sprintf('Missing @group annotation in %s', $classname));
+    }
+    $info['group'] = $annotations['group'];
+    $info['groups'] = $annotations['groups'];
+
+    // Sort out PHPUnit-runnable tests by type.
+    if ($testsuite = static::getPhpunitTestSuite($classname)) {
+      $info['type'] = 'PHPUnit-' . $testsuite;
+    }
+    else {
+      $info['type'] = 'Simpletest';
+    }
+
+    if (!empty($annotations['coversDefaultClass'])) {
+      $info['description'] = 'Tests ' . $annotations['coversDefaultClass'] . '.';
+    }
+    else {
+      $info['description'] = static::parseTestClassSummary($doc_comment);
+    }
+    if (isset($annotations['dependencies'])) {
+      $info['requires']['module'] = array_map('trim', explode(',', $annotations['dependencies']));
+    }
+
+    return $info;
+  }
+
+  /**
+   * Parses the phpDoc summary line of a test class.
+   *
+   * @param string $doc_comment
+   *
+   * @return string
+   *   The parsed phpDoc summary line. An empty string is returned if no summary
+   *   line can be parsed.
+   */
+  public static function parseTestClassSummary($doc_comment) {
+    // Normalize line endings.
+    $doc_comment = preg_replace('/\r\n|\r/', '\n', $doc_comment);
+    // Strip leading and trailing doc block lines.
+    $doc_comment = substr($doc_comment, 4, -4);
+
+    $lines = explode("\n", $doc_comment);
+    $summary = [];
+    // Add every line to the summary until the first empty line or annotation
+    // is found.
+    foreach ($lines as $line) {
+      if (preg_match('/^[ ]*\*$/', $line) || preg_match('/^[ ]*\* \@/', $line)) {
+        break;
+      }
+      $summary[] = trim($line, ' *');
+    }
+    return implode(' ', $summary);
+  }
+
+  /**
+   * Parses annotations in the phpDoc of a test class.
+   *
+   * @param \ReflectionClass $class
+   *   The reflected test class.
+   *
+   * @return array
+   *   An associative array that contains all annotations on the test class;
+   *   typically including:
+   *   - group: A list of @group values.
+   *   - requires: An associative array of @requires values; e.g.:
+   *     - module: A list of Drupal module dependencies that are required to
+   *       exist.
+   *
+   * @see PHPUnit_Util_Test::parseTestMethodAnnotations()
+   * @see http://phpunit.de/manual/current/en/incomplete-and-skipped-tests.html#incomplete-and-skipped-tests.skipping-tests-using-requires
+   */
+  public static function parseTestClassAnnotations(\ReflectionClass $class) {
+    $annotations = PHPUnit_Util_Test::parseTestMethodAnnotations($class->getName())['class'];
+
+    // @todo Enhance PHPUnit upstream to allow for custom @requires identifiers.
+    // @see PHPUnit_Util_Test::getRequirements()
+    // @todo Add support for 'PHP', 'OS', 'function', 'extension'.
+    // @see https://www.drupal.org/node/1273478
+    if (isset($annotations['requires'])) {
+      foreach ($annotations['requires'] as $i => $value) {
+        list($type, $value) = explode(' ', $value, 2);
+        if ($type === 'module') {
+          $annotations['requires']['module'][$value] = $value;
+          unset($annotations['requires'][$i]);
+        }
+      }
+    }
+    return $annotations;
+  }
+
+  /**
+   * Determines the phpunit testsuite for a given classname, based on namespace.
+   *
+   * @param string $classname
+   *   The test classname.
+   *
+   * @return string|false
+   *   The testsuite name or FALSE if its not a phpunit test.
+   */
+  public static function getPhpunitTestSuite($classname) {
+    if (preg_match('/Drupal\\\\Tests\\\\(\w+)\\\\(\w+)/', $classname, $matches)) {
+      // This could be an extension test, in which case the first match will be
+      // the extension name. We assume that lower-case strings are module names.
+      if (strtolower($matches[1]) == $matches[1]) {
+        return $matches[2];
+      }
+      return 'Unit';
+    }
+    // Core tests.
+    elseif (preg_match('/Drupal\\\\(\w*)Tests\\\\/', $classname, $matches)) {
+      if ($matches[1] == '') {
+        return 'Unit';
+      }
+      return $matches[1];
+    }
+    return FALSE;
+  }
+
+  /**
+   * Returns all available extensions.
+   *
+   * @return \Drupal\Core\Extension\Extension[]
+   *   An array of Extension objects, keyed by extension name.
+   */
+  protected function getExtensions() {
+    $listing = new ExtensionDiscovery($this->root);
+    // Ensure that tests in all profiles are discovered.
+    $listing->setProfileDirectories([]);
+    $extensions = $listing->scan('module', TRUE);
+    $extensions += $listing->scan('profile', TRUE);
+    $extensions += $listing->scan('theme', TRUE);
+    return $extensions;
+  }
+
+}
diff --git a/core/lib/Drupal/Core/Test/TestRunnerKernel.php b/core/lib/Drupal/Core/Test/TestRunnerKernel.php
index b615ce356fa11fb06d5bbb61b90077fe2096d1f9..963a6c4e4d8628fc2fbba46e157e4eae9f72969d 100644
--- a/core/lib/Drupal/Core/Test/TestRunnerKernel.php
+++ b/core/lib/Drupal/Core/Test/TestRunnerKernel.php
@@ -70,7 +70,11 @@ public function boot() {
 
     $this->getContainer()->get('module_handler')->loadAll();
 
-    $this->getContainer()->get('test_discovery')->registerTestNamespaces();
+    $test_discovery = new TestDiscovery(
+      $this->getContainer()->get('app.root'),
+      $this->getContainer()->get('class_loader')
+    );
+    $test_discovery->registerTestNamespaces();
 
     // Register stream wrappers.
     $this->getContainer()->get('stream_wrapper_manager')->register();
diff --git a/core/modules/simpletest/simpletest.services.yml b/core/modules/simpletest/simpletest.services.yml
index 326cf38f2d371f3093ba82f3a57f40ff3e415fa9..241519795798a63bee19e260ea7f2bcc683afa7f 100644
--- a/core/modules/simpletest/simpletest.services.yml
+++ b/core/modules/simpletest/simpletest.services.yml
@@ -1,5 +1,7 @@
 services:
   test_discovery:
+    # @todo Change this service so that it uses \Drupal\Core\Test\TestDiscovery
+    # in https://www.drupal.org/node/1667822
     class: Drupal\simpletest\TestDiscovery
     arguments: ['@app.root', '@class_loader', '@module_handler']
   cache_context.test_discovery:
diff --git a/core/modules/simpletest/src/Exception/MissingGroupException.php b/core/modules/simpletest/src/Exception/MissingGroupException.php
index 85a0696ca16cbfc5e6b95148b75ba4b091876fc2..c320304c9e1a42114d75e51f1fa9e0d5096c8496 100644
--- a/core/modules/simpletest/src/Exception/MissingGroupException.php
+++ b/core/modules/simpletest/src/Exception/MissingGroupException.php
@@ -2,8 +2,17 @@
 
 namespace Drupal\simpletest\Exception;
 
+@trigger_error(__NAMESPACE__ . '\\MissingGroupException is deprecated in drupal:8.8.0 and is removed from drupal:9.0.0. Use \Drupal\Core\Test\Exception\MissingGroupException instead. See https://www.drupal.org/node/2949692', E_USER_DEPRECATED);
+
+use Drupal\Core\Test\Exception\MissingGroupException as CoreMissingGroupException;
+
 /**
  * Exception thrown when a simpletest class is missing an @group annotation.
+ *
+ * @deprecated in drupal:8.8.0 and is removed from drupal:9.0.0. Use
+ *   \Drupal\Core\Test\Exception\MissingGroupException instead.
+ *
+ * @see https://www.drupal.org/node/2949692
  */
-class MissingGroupException extends \LogicException {
+class MissingGroupException extends CoreMissingGroupException {
 }
diff --git a/core/modules/simpletest/src/TestBase.php b/core/modules/simpletest/src/TestBase.php
index fccb7893fd5fb9f87be51df09ac74b3e26c0d243..9856e7fe4b38e9b9ff98a9482f496f5f96fc3926 100644
--- a/core/modules/simpletest/src/TestBase.php
+++ b/core/modules/simpletest/src/TestBase.php
@@ -12,6 +12,7 @@
 use Drupal\Core\Site\Settings;
 use Drupal\Core\StreamWrapper\PublicStream;
 use Drupal\Core\Test\TestDatabase;
+use Drupal\Core\Test\TestDiscovery;
 use Drupal\Core\Test\TestSetupTrait;
 use Drupal\Core\Utility\Error;
 use Drupal\Tests\AssertHelperTrait as BaseAssertHelperTrait;
diff --git a/core/modules/simpletest/src/TestDiscovery.php b/core/modules/simpletest/src/TestDiscovery.php
index 39079e17f65f3835e89f17cec472e83afd2d6f6c..8ce8b7de464cbdc75b7257956733131c6e463382 100644
--- a/core/modules/simpletest/src/TestDiscovery.php
+++ b/core/modules/simpletest/src/TestDiscovery.php
@@ -2,53 +2,27 @@
 
 namespace Drupal\simpletest;
 
+@trigger_error(__NAMESPACE__ . '\\TestDiscovery is deprecated in drupal:8.8.0 and is removed from drupal:9.0.0. Use \Drupal\Core\Test\TestDiscovery instead. See https://www.drupal.org/node/2949692', E_USER_DEPRECATED);
+
 use Doctrine\Common\Reflection\StaticReflectionParser;
 use Drupal\Component\Annotation\Reflection\MockFileFinder;
 use Drupal\Component\Utility\NestedArray;
-use Drupal\Core\Extension\ExtensionDiscovery;
 use Drupal\Core\Extension\ModuleHandlerInterface;
-use Drupal\simpletest\Exception\MissingGroupException;
-use PHPUnit_Util_Test;
+use Drupal\Core\Test\Exception\MissingGroupException;
+use Drupal\Core\Test\TestDiscovery as CoreTestDiscovery;
 
 /**
  * Discovers available tests.
+ *
+ * This class provides backwards compatibility for code which uses the legacy
+ * \Drupal\simpletest\TestDiscovery.
+ *
+ * @deprecated in drupal:8.8.0 and is removed from drupal:9.0.0. Use
+ *   \Drupal\Core\Test\TestDiscovery instead.
+ *
+ * @see https://www.drupal.org/node/2949692
  */
-class TestDiscovery {
-
-  /**
-   * The class loader.
-   *
-   * @var \Composer\Autoload\ClassLoader
-   */
-  protected $classLoader;
-
-  /**
-   * Statically cached list of test classes.
-   *
-   * @var array
-   */
-  protected $testClasses;
-
-  /**
-   * Cached map of all test namespaces to respective directories.
-   *
-   * @var array
-   */
-  protected $testNamespaces;
-
-  /**
-   * Cached list of all available extension names, keyed by extension type.
-   *
-   * @var array
-   */
-  protected $availableExtensions;
-
-  /**
-   * The app root.
-   *
-   * @var string
-   */
-  protected $root;
+class TestDiscovery extends CoreTestDiscovery {
 
   /**
    * The module handler.
@@ -70,66 +44,17 @@ class TestDiscovery {
    *   The module handler.
    */
   public function __construct($root, $class_loader, ModuleHandlerInterface $module_handler) {
-    $this->root = $root;
-    $this->classLoader = $class_loader;
+    parent::__construct($root, $class_loader);
     $this->moduleHandler = $module_handler;
   }
 
-  /**
-   * Registers test namespaces of all extensions and core test classes.
-   *
-   * @return array
-   *   An associative array whose keys are PSR-4 namespace prefixes and whose
-   *   values are directory names.
-   */
-  public function registerTestNamespaces() {
-    if (isset($this->testNamespaces)) {
-      return $this->testNamespaces;
-    }
-    $this->testNamespaces = [];
-
-    $existing = $this->classLoader->getPrefixesPsr4();
-
-    // Add PHPUnit test namespaces of Drupal core.
-    $this->testNamespaces['Drupal\\Tests\\'] = [$this->root . '/core/tests/Drupal/Tests'];
-    $this->testNamespaces['Drupal\\KernelTests\\'] = [$this->root . '/core/tests/Drupal/KernelTests'];
-    $this->testNamespaces['Drupal\\FunctionalTests\\'] = [$this->root . '/core/tests/Drupal/FunctionalTests'];
-    $this->testNamespaces['Drupal\\FunctionalJavascriptTests\\'] = [$this->root . '/core/tests/Drupal/FunctionalJavascriptTests'];
-
-    $this->availableExtensions = [];
-    foreach ($this->getExtensions() as $name => $extension) {
-      $this->availableExtensions[$extension->getType()][$name] = $name;
-
-      $base_path = $this->root . '/' . $extension->getPath();
-
-      // Add namespace of disabled/uninstalled extensions.
-      if (!isset($existing["Drupal\\$name\\"])) {
-        $this->classLoader->addPsr4("Drupal\\$name\\", "$base_path/src");
-      }
-      // Add Simpletest test namespace.
-      $this->testNamespaces["Drupal\\$name\\Tests\\"][] = "$base_path/src/Tests";
-
-      // Add PHPUnit test namespaces.
-      $this->testNamespaces["Drupal\\Tests\\$name\\Unit\\"][] = "$base_path/tests/src/Unit";
-      $this->testNamespaces["Drupal\\Tests\\$name\\Kernel\\"][] = "$base_path/tests/src/Kernel";
-      $this->testNamespaces["Drupal\\Tests\\$name\\Functional\\"][] = "$base_path/tests/src/Functional";
-      $this->testNamespaces["Drupal\\Tests\\$name\\FunctionalJavascript\\"][] = "$base_path/tests/src/FunctionalJavascript";
-
-      // Add discovery for traits which are shared between different test
-      // suites.
-      $this->testNamespaces["Drupal\\Tests\\$name\\Traits\\"][] = "$base_path/tests/src/Traits";
-    }
-
-    foreach ($this->testNamespaces as $prefix => $paths) {
-      $this->classLoader->addPsr4($prefix, $paths);
-    }
-
-    return $this->testNamespaces;
-  }
-
   /**
    * Discovers all available tests in all extensions.
    *
+   * This method is a near-duplicate of
+   * \Drupal\Core\Tests\TestDiscovery::getTestClasses(). It exists so that we
+   * can provide a BC invocation of hook_simpletest_alter().
+   *
    * @param string $extension
    *   (optional) The name of an extension to limit discovery to; e.g., 'node'.
    * @param string[] $types
@@ -149,9 +74,6 @@ public function registerTestNamespaces() {
    *       ),
    *     );
    * @endcode
-   *
-   * @todo Remove singular grouping; retain list of groups in 'group' key.
-   * @see https://www.drupal.org/node/2296615
    */
   public function getTestClasses($extension = NULL, array $types = []) {
     if (!isset($extension) && empty($types)) {
@@ -225,277 +147,4 @@ public function getTestClasses($extension = NULL, array $types = []) {
     return $list;
   }
 
-  /**
-   * Discovers all class files in all available extensions.
-   *
-   * @param string $extension
-   *   (optional) The name of an extension to limit discovery to; e.g., 'node'.
-   *
-   * @return array
-   *   A classmap containing all discovered class files; i.e., a map of
-   *   fully-qualified classnames to pathnames.
-   */
-  public function findAllClassFiles($extension = NULL) {
-    $classmap = [];
-    $namespaces = $this->registerTestNamespaces();
-    if (isset($extension)) {
-      // Include tests in the \Drupal\Tests\{$extension} namespace.
-      $pattern = "/Drupal\\\(Tests\\\)?$extension\\\/";
-      $namespaces = array_intersect_key($namespaces, array_flip(preg_grep($pattern, array_keys($namespaces))));
-    }
-    foreach ($namespaces as $namespace => $paths) {
-      foreach ($paths as $path) {
-        if (!is_dir($path)) {
-          continue;
-        }
-        $classmap += static::scanDirectory($namespace, $path);
-      }
-    }
-    return $classmap;
-  }
-
-  /**
-   * Scans a given directory for class files.
-   *
-   * @param string $namespace_prefix
-   *   The namespace prefix to use for discovered classes. Must contain a
-   *   trailing namespace separator (backslash).
-   *   For example: 'Drupal\\node\\Tests\\'
-   * @param string $path
-   *   The directory path to scan.
-   *   For example: '/path/to/drupal/core/modules/node/tests/src'
-   *
-   * @return array
-   *   An associative array whose keys are fully-qualified class names and whose
-   *   values are corresponding filesystem pathnames.
-   *
-   * @throws \InvalidArgumentException
-   *   If $namespace_prefix does not end in a namespace separator (backslash).
-   *
-   * @todo Limit to '*Test.php' files (~10% less files to reflect/introspect).
-   * @see https://www.drupal.org/node/2296635
-   */
-  public static function scanDirectory($namespace_prefix, $path) {
-    if (substr($namespace_prefix, -1) !== '\\') {
-      throw new \InvalidArgumentException("Namespace prefix for $path must contain a trailing namespace separator.");
-    }
-    $flags = \FilesystemIterator::UNIX_PATHS;
-    $flags |= \FilesystemIterator::SKIP_DOTS;
-    $flags |= \FilesystemIterator::FOLLOW_SYMLINKS;
-    $flags |= \FilesystemIterator::CURRENT_AS_SELF;
-    $flags |= \FilesystemIterator::KEY_AS_FILENAME;
-
-    $iterator = new \RecursiveDirectoryIterator($path, $flags);
-    $filter = new \RecursiveCallbackFilterIterator($iterator, function ($current, $file_name, $iterator) {
-      if ($iterator->hasChildren()) {
-        return TRUE;
-      }
-      // We don't want to discover abstract TestBase classes, traits or
-      // interfaces. They can be deprecated and will call @trigger_error()
-      // during discovery.
-      return substr($file_name, -4) === '.php' &&
-        substr($file_name, -12) !== 'TestBase.php' &&
-        substr($file_name, -9) !== 'Trait.php' &&
-        substr($file_name, -13) !== 'Interface.php';
-    });
-    $files = new \RecursiveIteratorIterator($filter);
-    $classes = [];
-    foreach ($files as $fileinfo) {
-      $class = $namespace_prefix;
-      if ('' !== $subpath = $fileinfo->getSubPath()) {
-        $class .= strtr($subpath, '/', '\\') . '\\';
-      }
-      $class .= $fileinfo->getBasename('.php');
-      $classes[$class] = $fileinfo->getPathname();
-    }
-    return $classes;
-  }
-
-  /**
-   * Retrieves information about a test class for UI purposes.
-   *
-   * @param string $classname
-   *   The test classname.
-   * @param string $doc_comment
-   *   (optional) The class PHPDoc comment. If not passed in reflection will be
-   *   used but this is very expensive when parsing all the test classes.
-   *
-   * @return array
-   *   An associative array containing:
-   *   - name: The test class name.
-   *   - description: The test (PHPDoc) summary.
-   *   - group: The test's first @group (parsed from PHPDoc annotations).
-   *   - groups: All of the test's @group annotations, as an array (parsed from
-   *     PHPDoc annotations).
-   *   - requires: An associative array containing test requirements parsed from
-   *     PHPDoc annotations:
-   *     - module: List of Drupal module extension names the test depends on.
-   *
-   * @throws \Drupal\simpletest\Exception\MissingGroupException
-   *   If the class does not have a @group annotation.
-   */
-  public static function getTestInfo($classname, $doc_comment = NULL) {
-    if ($doc_comment === NULL) {
-      $reflection = new \ReflectionClass($classname);
-      $doc_comment = $reflection->getDocComment();
-    }
-    $info = [
-      'name' => $classname,
-    ];
-    $annotations = [];
-    // Look for annotations, allow an arbitrary amount of spaces before the
-    // * but nothing else.
-    preg_match_all('/^[ ]*\* \@([^\s]*) (.*$)/m', $doc_comment, $matches);
-    if (isset($matches[1])) {
-      foreach ($matches[1] as $key => $annotation) {
-        // For historical reasons, there is a single-value 'group' result key
-        // and a 'groups' key as an array.
-        if ($annotation === 'group') {
-          $annotations['groups'][] = $matches[2][$key];
-        }
-        if (!empty($annotations[$annotation])) {
-          // Only @group is allowed to have more than one annotation, in the
-          // 'groups' key. Other annotations only have one value per key.
-          continue;
-        }
-        $annotations[$annotation] = $matches[2][$key];
-      }
-    }
-
-    if (empty($annotations['group'])) {
-      // Concrete tests must have a group.
-      throw new MissingGroupException(sprintf('Missing @group annotation in %s', $classname));
-    }
-    $info['group'] = $annotations['group'];
-    $info['groups'] = $annotations['groups'];
-
-    // Sort out PHPUnit-runnable tests by type.
-    if ($testsuite = static::getPhpunitTestSuite($classname)) {
-      $info['type'] = 'PHPUnit-' . $testsuite;
-    }
-    else {
-      $info['type'] = 'Simpletest';
-    }
-
-    if (!empty($annotations['coversDefaultClass'])) {
-      $info['description'] = 'Tests ' . $annotations['coversDefaultClass'] . '.';
-    }
-    else {
-      $info['description'] = static::parseTestClassSummary($doc_comment);
-    }
-    if (isset($annotations['dependencies'])) {
-      $info['requires']['module'] = array_map('trim', explode(',', $annotations['dependencies']));
-    }
-
-    return $info;
-  }
-
-  /**
-   * Parses the phpDoc summary line of a test class.
-   *
-   * @param string $doc_comment
-   *
-   * @return string
-   *   The parsed phpDoc summary line. An empty string is returned if no summary
-   *   line can be parsed.
-   */
-  public static function parseTestClassSummary($doc_comment) {
-    // Normalize line endings.
-    $doc_comment = preg_replace('/\r\n|\r/', '\n', $doc_comment);
-    // Strip leading and trailing doc block lines.
-    $doc_comment = substr($doc_comment, 4, -4);
-
-    $lines = explode("\n", $doc_comment);
-    $summary = [];
-    // Add every line to the summary until the first empty line or annotation
-    // is found.
-    foreach ($lines as $line) {
-      if (preg_match('/^[ ]*\*$/', $line) || preg_match('/^[ ]*\* \@/', $line)) {
-        break;
-      }
-      $summary[] = trim($line, ' *');
-    }
-    return implode(' ', $summary);
-  }
-
-  /**
-   * Parses annotations in the phpDoc of a test class.
-   *
-   * @param \ReflectionClass $class
-   *   The reflected test class.
-   *
-   * @return array
-   *   An associative array that contains all annotations on the test class;
-   *   typically including:
-   *   - group: A list of @group values.
-   *   - requires: An associative array of @requires values; e.g.:
-   *     - module: A list of Drupal module dependencies that are required to
-   *       exist.
-   *
-   * @see PHPUnit_Util_Test::parseTestMethodAnnotations()
-   * @see http://phpunit.de/manual/current/en/incomplete-and-skipped-tests.html#incomplete-and-skipped-tests.skipping-tests-using-requires
-   */
-  public static function parseTestClassAnnotations(\ReflectionClass $class) {
-    $annotations = PHPUnit_Util_Test::parseTestMethodAnnotations($class->getName())['class'];
-
-    // @todo Enhance PHPUnit upstream to allow for custom @requires identifiers.
-    // @see PHPUnit_Util_Test::getRequirements()
-    // @todo Add support for 'PHP', 'OS', 'function', 'extension'.
-    // @see https://www.drupal.org/node/1273478
-    if (isset($annotations['requires'])) {
-      foreach ($annotations['requires'] as $i => $value) {
-        list($type, $value) = explode(' ', $value, 2);
-        if ($type === 'module') {
-          $annotations['requires']['module'][$value] = $value;
-          unset($annotations['requires'][$i]);
-        }
-      }
-    }
-    return $annotations;
-  }
-
-  /**
-   * Determines the phpunit testsuite for a given classname, based on namespace.
-   *
-   * @param string $classname
-   *   The test classname.
-   *
-   * @return string|false
-   *   The testsuite name or FALSE if its not a phpunit test.
-   */
-  public static function getPhpunitTestSuite($classname) {
-    if (preg_match('/Drupal\\\\Tests\\\\(\w+)\\\\(\w+)/', $classname, $matches)) {
-      // This could be an extension test, in which case the first match will be
-      // the extension name. We assume that lower-case strings are module names.
-      if (strtolower($matches[1]) == $matches[1]) {
-        return $matches[2];
-      }
-      return 'Unit';
-    }
-    // Core tests.
-    elseif (preg_match('/Drupal\\\\(\w*)Tests\\\\/', $classname, $matches)) {
-      if ($matches[1] == '') {
-        return 'Unit';
-      }
-      return $matches[1];
-    }
-    return FALSE;
-  }
-
-  /**
-   * Returns all available extensions.
-   *
-   * @return \Drupal\Core\Extension\Extension[]
-   *   An array of Extension objects, keyed by extension name.
-   */
-  protected function getExtensions() {
-    $listing = new ExtensionDiscovery($this->root);
-    // Ensure that tests in all profiles are discovered.
-    $listing->setProfileDirectories([]);
-    $extensions = $listing->scan('module', TRUE);
-    $extensions += $listing->scan('profile', TRUE);
-    $extensions += $listing->scan('theme', TRUE);
-    return $extensions;
-  }
-
 }
diff --git a/core/modules/simpletest/src/Tests/BrokenSetUpTest.php b/core/modules/simpletest/src/Tests/BrokenSetUpTest.php
index 28fd31beb3653d2194064a32831e670afbd1f0c9..fab44d00c820b62e54c823ec02bbe344f998f433 100644
--- a/core/modules/simpletest/src/Tests/BrokenSetUpTest.php
+++ b/core/modules/simpletest/src/Tests/BrokenSetUpTest.php
@@ -13,6 +13,8 @@
  * properly are skipped.
  *
  * @group WebTestBase
+ * @group legacy
+ *
  * @see \Drupal\simpletest\WebTestBase
  */
 class BrokenSetUpTest extends WebTestBase {
diff --git a/core/modules/simpletest/src/Tests/MissingCheckedRequirementsTest.php b/core/modules/simpletest/src/Tests/MissingCheckedRequirementsTest.php
index ae9d6f7291ceb3dcef6f36af1505300154ea8980..4bf7c7b6cc0ddf678e298a6e757f495f90164c7e 100644
--- a/core/modules/simpletest/src/Tests/MissingCheckedRequirementsTest.php
+++ b/core/modules/simpletest/src/Tests/MissingCheckedRequirementsTest.php
@@ -9,6 +9,7 @@
  *
  * @group simpletest
  * @group WebTestBase
+ * @group legacy
  */
 class MissingCheckedRequirementsTest extends WebTestBase {
 
diff --git a/core/modules/simpletest/src/Tests/SimpleTestTest.php b/core/modules/simpletest/src/Tests/SimpleTestTest.php
index c26fbdcbfeec9e77cad04c85db53b53dedb370cd..ea10bcef71249be41b572b9ed2578d3c47c45afb 100644
--- a/core/modules/simpletest/src/Tests/SimpleTestTest.php
+++ b/core/modules/simpletest/src/Tests/SimpleTestTest.php
@@ -14,6 +14,7 @@
  *
  * @group simpletest
  * @group WebTestBase
+ * @group legacy
  */
 class SimpleTestTest extends WebTestBase {
 
diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index 0d7cd953ae3c262faeb8a6b30163b98d556ccc36..3f0ab2ff131f20198671541f4ba0d3a28f7bb8ba 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -14,6 +14,7 @@
 use Drupal\Core\Session\AnonymousUserSession;
 use Drupal\Core\Test\AssertMailTrait;
 use Drupal\Core\Test\FunctionalTestSetupTrait;
+use Drupal\Core\Test\TestDiscovery;
 use Drupal\Core\Url;
 use Drupal\KernelTests\AssertContentTrait as CoreAssertContentTrait;
 use Drupal\system\Tests\Cache\AssertPageCacheContextsAndTagsTrait;
@@ -705,7 +706,7 @@ protected function curlHeaderCallback($curlHandler, $header) {
         if (getenv('SYMFONY_DEPRECATIONS_HELPER') !== 'disabled') {
           $message = (string) $parameters[0];
           $test_info = TestDiscovery::getTestInfo(get_called_class());
-          if ($test_info['group'] !== 'legacy' && !in_array($message, DeprecationListenerTrait::getSkippedDeprecations())) {
+          if (!in_array('legacy', $test_info['groups']) && !in_array($message, DeprecationListenerTrait::getSkippedDeprecations())) {
             call_user_func_array([&$this, 'error'], $parameters);
           }
         }
diff --git a/core/modules/simpletest/tests/src/Functional/OtherInstallationProfileTestsTest.php b/core/modules/simpletest/tests/src/Functional/OtherInstallationProfileTestsTest.php
index 972495f575962b1ac41c22aeb600c7503d82b5d7..8a9ba18281a0359696291d328b982ee285054721 100644
--- a/core/modules/simpletest/tests/src/Functional/OtherInstallationProfileTestsTest.php
+++ b/core/modules/simpletest/tests/src/Functional/OtherInstallationProfileTestsTest.php
@@ -3,11 +3,14 @@
 namespace Drupal\Tests\simpletest\Functional;
 
 use Drupal\Tests\BrowserTestBase;
+use Drupal\Core\Url;
 
 /**
  * Verifies that tests in other installation profiles are found.
  *
  * @group simpletest
+ * @group legacy
+ *
  * @see \Drupal\simpletest\Tests\InstallationProfileModuleTestsTest
  */
 class OtherInstallationProfileTestsTest extends BrowserTestBase {
@@ -51,11 +54,13 @@ protected function setUp() {
 
   /**
    * Tests that tests located in another installation profile appear.
+   *
+   * @expectedDeprecation Drupal\simpletest\TestDiscovery is deprecated in drupal:8.8.0 and is removed from drupal:9.0.0. Use \Drupal\Core\Test\TestDiscovery instead. See https://www.drupal.org/node/2949692
    */
   public function testOtherInstallationProfile() {
     // Assert the existence of a test in a different installation profile than
     // the current.
-    $this->drupalGet('admin/config/development/testing');
+    $this->drupalGet(Url::fromRoute('simpletest.test_form'));
     $this->assertText('Tests Standard installation profile expectations.');
 
     // Assert the existence of a test for a module in a different installation
diff --git a/core/modules/simpletest/tests/src/Functional/SimpletestUiTest.php b/core/modules/simpletest/tests/src/Functional/SimpletestUiTest.php
index f91cb6d5077dc6cf229445d0a70e328d029c44b2..9103cb8aad6dab712f5591ad342109610b7b2cd9 100644
--- a/core/modules/simpletest/tests/src/Functional/SimpletestUiTest.php
+++ b/core/modules/simpletest/tests/src/Functional/SimpletestUiTest.php
@@ -12,6 +12,7 @@
  *
  * @group #slow
  * @group simpletest
+ * @group legacy
  */
 class SimpletestUiTest extends BrowserTestBase {
 
@@ -30,6 +31,8 @@ protected function setUp() {
 
   /**
    * Tests that unit, kernel, and functional tests work through the UI.
+   *
+   * @expectedDeprecation Drupal\simpletest\TestDiscovery is deprecated in drupal:8.8.0 and is removed from drupal:9.0.0. Use \Drupal\Core\Test\TestDiscovery instead. See https://www.drupal.org/node/2949692
    */
   public function testTestingThroughUI() {
     $url = Url::fromRoute('simpletest.test_form');
diff --git a/core/modules/simpletest/tests/src/Functional/ThroughUITest.php b/core/modules/simpletest/tests/src/Functional/ThroughUITest.php
index 48737433f366d208d907548f1ff1eb411342a654..1e325e4c55241656442b5adf996ffbada2fc8297 100644
--- a/core/modules/simpletest/tests/src/Functional/ThroughUITest.php
+++ b/core/modules/simpletest/tests/src/Functional/ThroughUITest.php
@@ -10,6 +10,7 @@
  * @see \Drupal\simpletest\Tests::testTestingThroughUI()
  *
  * @group simpletest
+ * @group legacy
  */
 class ThroughUITest extends BrowserTestBase {
 
diff --git a/core/modules/simpletest/tests/src/Kernel/Cache/Context/TestDiscoveryCacheContextTest.php b/core/modules/simpletest/tests/src/Kernel/Cache/Context/TestDiscoveryCacheContextTest.php
index 25b2af92e4398aba6c4f7ba6fa96994ccb990409..00d83581cfd66f1c5160975fe56f69df90bbff79 100644
--- a/core/modules/simpletest/tests/src/Kernel/Cache/Context/TestDiscoveryCacheContextTest.php
+++ b/core/modules/simpletest/tests/src/Kernel/Cache/Context/TestDiscoveryCacheContextTest.php
@@ -8,6 +8,7 @@
 
 /**
  * @group simpletest
+ * @group legacy
  */
 class TestDiscoveryCacheContextTest extends KernelTestBase {
 
@@ -18,6 +19,8 @@ class TestDiscoveryCacheContextTest extends KernelTestBase {
 
   /**
    * Tests that test context hashes are unique.
+   *
+   * @expectedDeprecation Drupal\simpletest\TestDiscovery is deprecated in drupal:8.8.0 and is removed from drupal:9.0.0. Use \Drupal\Core\Test\TestDiscovery instead. See https://www.drupal.org/node/2949692
    */
   public function testContext() {
     // Mock test discovery.
diff --git a/core/modules/simpletest/tests/src/Kernel/SimpletestDeprecationTest.php b/core/modules/simpletest/tests/src/Kernel/SimpletestDeprecationTest.php
index 384f8c1b9b36a79d04a379dc05a5303a4edc0255..36c3d43dc7c2af336579e304813d7d64b25e1a42 100644
--- a/core/modules/simpletest/tests/src/Kernel/SimpletestDeprecationTest.php
+++ b/core/modules/simpletest/tests/src/Kernel/SimpletestDeprecationTest.php
@@ -3,9 +3,10 @@
 namespace Drupal\Tests\simpletest\Kernel;
 
 use Drupal\KernelTests\KernelTestBase;
+use Drupal\simpletest\TestDiscovery;
 
 /**
- * Verify deprecation of simpletest.
+ * Verify deprecations within the simpletest module.
  *
  * @group simpletest
  * @group legacy
@@ -25,4 +26,12 @@ public function testDeprecatedFunctions() {
     simpletest_classloader_register();
   }
 
+  /**
+   * @expectedDeprecation Drupal\simpletest\TestDiscovery is deprecated in drupal:8.8.0 and is removed from drupal:9.0.0. Use \Drupal\Core\Test\TestDiscovery instead. See https://www.drupal.org/node/2949692
+   * @expectedDeprecation The "test_discovery" service relies on the deprecated "Drupal\simpletest\TestDiscovery" class. It should either be deprecated or its implementation upgraded.
+   */
+  public function testDeprecatedServices() {
+    $this->assertInstanceOf(TestDiscovery::class, $this->container->get('test_discovery'));
+  }
+
 }
diff --git a/core/modules/simpletest/tests/src/Kernel/TestDiscoveryDeprecationTest.php b/core/modules/simpletest/tests/src/Kernel/TestDiscoveryDeprecationTest.php
index 6d4f44888a046f8d6dc182d015512c45d2791eff..c9c781d278cabaa32bbcf7f770c22709507bb07a 100644
--- a/core/modules/simpletest/tests/src/Kernel/TestDiscoveryDeprecationTest.php
+++ b/core/modules/simpletest/tests/src/Kernel/TestDiscoveryDeprecationTest.php
@@ -3,6 +3,7 @@
 namespace Drupal\Tests\simpletest\Kernel;
 
 use Drupal\KernelTests\KernelTestBase;
+use Drupal\simpletest\TestDiscovery;
 
 /**
  * @group simpletest
@@ -22,11 +23,13 @@ class TestDiscoveryDeprecationTest extends KernelTestBase {
    * @covers ::getTestClasses
    */
   public function testHookSimpletestAlter() {
+    $test_discovery = $this->container->get('test_discovery');
+
+    $this->assertEquals(TestDiscovery::class, get_class($test_discovery));
+
     // The simpletest_test module implements hook_simpletest_alter(), which
     // should trigger a deprecation error during getTestClasses().
-    $this->assertNotEmpty(
-      $this->container->get('test_discovery')->getTestClasses()
-    );
+    $this->assertNotEmpty($test_discovery->getTestClasses());
   }
 
 }
diff --git a/core/modules/simpletest/tests/src/Unit/TestDiscoveryTest.php b/core/modules/simpletest/tests/src/Unit/TestDiscoveryTest.php
index 588260acc512a991e0dbc0b3e1e422c82d30d2ab..477f74fbe732e04edf0a0752f5f4144cdc8b065c 100644
--- a/core/modules/simpletest/tests/src/Unit/TestDiscoveryTest.php
+++ b/core/modules/simpletest/tests/src/Unit/TestDiscoveryTest.php
@@ -7,296 +7,17 @@
 use Drupal\Core\DrupalKernel;
 use Drupal\Core\Extension\Extension;
 use Drupal\Core\Extension\ModuleHandlerInterface;
-use Drupal\simpletest\Exception\MissingGroupException;
 use Drupal\simpletest\TestDiscovery;
 use Drupal\Tests\UnitTestCase;
 use org\bovigo\vfs\vfsStream;
 
 /**
  * @coversDefaultClass \Drupal\simpletest\TestDiscovery
- * @group simpletest
- */
-class TestDiscoveryTest extends UnitTestCase {
-
-  /**
-   * @covers ::getTestInfo
-   * @dataProvider infoParserProvider
-   */
-  public function testTestInfoParser($expected, $classname, $doc_comment = NULL) {
-    $info = TestDiscovery::getTestInfo($classname, $doc_comment);
-    $this->assertEquals($expected, $info);
-  }
-
-  public function infoParserProvider() {
-    // A module provided unit test.
-    $tests[] = [
-      // Expected result.
-      [
-        'name' => static::class,
-        'group' => 'simpletest',
-        'groups' => ['simpletest'],
-        'description' => 'Tests \Drupal\simpletest\TestDiscovery.',
-        'type' => 'PHPUnit-Unit',
-      ],
-      // Classname.
-      static::class,
-    ];
-
-    // A core unit test.
-    $tests[] = [
-      // Expected result.
-      [
-        'name' => 'Drupal\Tests\Core\DrupalTest',
-        'group' => 'DrupalTest',
-        'groups' => ['DrupalTest'],
-        'description' => 'Tests \Drupal.',
-        'type' => 'PHPUnit-Unit',
-      ],
-      // Classname.
-      'Drupal\Tests\Core\DrupalTest',
-    ];
-
-    // Functional PHPUnit test.
-    $tests[] = [
-      // Expected result.
-      [
-        'name' => 'Drupal\FunctionalTests\BrowserTestBaseTest',
-        'group' => 'browsertestbase',
-        'groups' => ['browsertestbase'],
-        'description' => 'Tests BrowserTestBase functionality.',
-        'type' => 'PHPUnit-Functional',
-      ],
-      // Classname.
-      'Drupal\FunctionalTests\BrowserTestBaseTest',
-    ];
-
-    // kernel PHPUnit test.
-    $tests['phpunit-kernel'] = [
-      // Expected result.
-      [
-        'name' => '\Drupal\Tests\file\Kernel\FileItemValidationTest',
-        'group' => 'file',
-        'groups' => ['file'],
-        'description' => 'Tests that files referenced in file and image fields are always validated.',
-        'type' => 'PHPUnit-Kernel',
-      ],
-      // Classname.
-      '\Drupal\Tests\file\Kernel\FileItemValidationTest',
-    ];
-
-    // Simpletest classes can not be autoloaded in a PHPUnit test, therefore
-    // provide a docblock.
-    $tests[] = [
-      // Expected result.
-      [
-        'name' => 'Drupal\simpletest\Tests\ExampleSimpleTest',
-        'group' => 'simpletest',
-        'groups' => ['simpletest'],
-        'description' => 'Tests the Simpletest UI internal browser.',
-        'type' => 'Simpletest',
-      ],
-      // Classname.
-      'Drupal\simpletest\Tests\ExampleSimpleTest',
-      // Doc block.
-      "/**
- * Tests the Simpletest UI internal browser.
- *
- * @group simpletest
- */
- ",
-    ];
-
-    // Test with a different amount of leading spaces.
-    $tests[] = [
-      // Expected result.
-      [
-        'name' => 'Drupal\simpletest\Tests\ExampleSimpleTest',
-        'group' => 'simpletest',
-        'groups' => ['simpletest'],
-        'description' => 'Tests the Simpletest UI internal browser.',
-        'type' => 'Simpletest',
-      ],
-      // Classname.
-      'Drupal\simpletest\Tests\ExampleSimpleTest',
-      // Doc block.
-      "/**
-   * Tests the Simpletest UI internal browser.
-   *
-   * @group simpletest
-   */
-   */
- ",
-    ];
-
-    // Make sure that a "* @" inside a string does not get parsed as an
-    // annotation.
-    $tests[] = [
-      // Expected result.
-      [
-        'name' => 'Drupal\simpletest\Tests\ExampleSimpleTest',
-        'group' => 'simpletest',
-        'groups' => ['simpletest'],
-        'description' => 'Tests the Simpletest UI internal browser. * @',
-        'type' => 'Simpletest',
-      ],
-      // Classname.
-      'Drupal\simpletest\Tests\ExampleSimpleTest',
-      // Doc block.
-      "/**
-   * Tests the Simpletest UI internal browser. * @
-   *
-   * @group simpletest
-   */
- ",
-    ];
-
-    // Multiple @group annotations.
-    $tests[] = [
-      // Expected result.
-      [
-        'name' => 'Drupal\simpletest\Tests\ExampleSimpleTest',
-        'group' => 'Test',
-        'groups' => ['Test', 'simpletest'],
-        'description' => 'Tests the Simpletest UI internal browser.',
-        'type' => 'Simpletest',
-      ],
-      // Classname.
-      'Drupal\simpletest\Tests\ExampleSimpleTest',
-      // Doc block.
-      "/**
- * Tests the Simpletest UI internal browser.
- *
- * @group Test
- * @group simpletest
- */
- ",
-    ];
-
-    // A great number of @group annotations.
-    $tests['many-group-annotations'] = [
-      // Expected result.
-      [
-        'name' => 'Drupal\simpletest\Tests\ExampleSimpleTest',
-        'group' => 'Test',
-        'groups' => ['Test', 'simpletest', 'another', 'more', 'many', 'enough', 'whoa'],
-        'description' => 'Tests the Simpletest UI internal browser.',
-        'type' => 'Simpletest',
-      ],
-      // Classname.
-      'Drupal\simpletest\Tests\ExampleSimpleTest',
-      // Doc block.
-      "/**
- * Tests the Simpletest UI internal browser.
- *
- * @group Test
- * @group simpletest
- * @group another
- * @group more
- * @group many
- * @group enough
- * @group whoa
- */
- ",
-    ];
-
-    // @dependencies annotation.
-    $tests[] = [
-      // Expected result.
-      [
-        'name' => 'Drupal\simpletest\Tests\ExampleSimpleTest',
-        'description' => 'Tests the Simpletest UI internal browser.',
-        'type' => 'Simpletest',
-        'requires' => ['module' => ['test']],
-        'group' => 'simpletest',
-        'groups' => ['simpletest'],
-      ],
-      // Classname.
-      'Drupal\simpletest\Tests\ExampleSimpleTest',
-      // Doc block.
-      "/**
- * Tests the Simpletest UI internal browser.
- *
- * @dependencies test
- * @group simpletest
- */
- ",
-    ];
-
-    // Multiple @dependencies annotation.
-    $tests[] = [
-      // Expected result.
-      [
-        'name' => 'Drupal\simpletest\Tests\ExampleSimpleTest',
-        'description' => 'Tests the Simpletest UI internal browser.',
-        'type' => 'Simpletest',
-        'requires' => ['module' => ['test', 'test1', 'test2']],
-        'group' => 'simpletest',
-        'groups' => ['simpletest'],
-      ],
-      // Classname.
-      'Drupal\simpletest\Tests\ExampleSimpleTest',
-      // Doc block.
-      "/**
- * Tests the Simpletest UI internal browser.
- *
- * @dependencies test, test1, test2
- * @group simpletest
- */
- ",
-    ];
-
-    // Multi-line summary line.
-    $tests[] = [
-      // Expected result.
-      [
-        'name' => 'Drupal\simpletest\Tests\ExampleSimpleTest',
-        'description' => 'Tests the Simpletest UI internal browser. And the summary line continues an there is no gap to the annotation.',
-        'type' => 'Simpletest',
-        'group' => 'simpletest',
-        'groups' => ['simpletest'],
-      ],
-      // Classname.
-      'Drupal\simpletest\Tests\ExampleSimpleTest',
-      // Doc block.
-      "/**
- * Tests the Simpletest UI internal browser. And the summary line continues an
- * there is no gap to the annotation.
  *
  * @group simpletest
+ * @group legacy
  */
- ",
-    ];
-    return $tests;
-  }
-
-  /**
-   * @covers ::getTestInfo
-   */
-  public function testTestInfoParserMissingGroup() {
-    $classname = 'Drupal\KernelTests\field\BulkDeleteTest';
-    $doc_comment = <<<EOT
-/**
- * Bulk delete storages and fields, and clean up afterwards.
- */
-EOT;
-    $this->expectException(MissingGroupException::class);
-    $this->expectExceptionMessage('Missing @group annotation in Drupal\KernelTests\field\BulkDeleteTest');
-    TestDiscovery::getTestInfo($classname, $doc_comment);
-  }
-
-  /**
-   * @covers ::getTestInfo
-   */
-  public function testTestInfoParserMissingSummary() {
-    $classname = 'Drupal\KernelTests\field\BulkDeleteTest';
-    $doc_comment = <<<EOT
-/**
- * @group field
- */
-EOT;
-    $info = TestDiscovery::getTestInfo($classname, $doc_comment);
-    $this->assertEmpty($info['description']);
-  }
+class TestDiscoveryTest extends UnitTestCase {
 
   protected function setupVfsWithTestClasses() {
     vfsStream::setup('drupal');
@@ -363,19 +84,34 @@ class FunctionalExampleTest {}
   }
 
   /**
-   * @covers ::getTestClasses
+   * Mock a TestDiscovery object to return specific extension values.
    */
-  public function testGetTestClasses() {
-    $this->setupVfsWithTestClasses();
+  protected function getTestDiscoveryMock($app_root, $extensions) {
     $class_loader = $this->prophesize(ClassLoader::class);
     $module_handler = $this->prophesize(ModuleHandlerInterface::class);
 
-    $test_discovery = new TestTestDiscovery('vfs://drupal', $class_loader->reveal(), $module_handler->reveal());
+    $test_discovery = $this->getMockBuilder(TestDiscovery::class)
+      ->setConstructorArgs([$app_root, $class_loader->reveal(), $module_handler->reveal()])
+      ->setMethods(['getExtensions'])
+      ->getMock();
+
+    $test_discovery->expects($this->any())
+      ->method('getExtensions')
+      ->willReturn($extensions);
 
+    return $test_discovery;
+  }
+
+  /**
+   * @covers ::getTestClasses
+   */
+  public function testGetTestClasses() {
+    $this->setupVfsWithTestClasses();
     $extensions = [
       'test_module' => new Extension('vfs://drupal', 'module', 'modules/test_module/test_module.info.yml'),
     ];
-    $test_discovery->setExtensions($extensions);
+    $test_discovery = $this->getTestDiscoveryMock('vfs://drupal', $extensions);
+
     $result = $test_discovery->getTestClasses();
     $this->assertCount(3, $result);
     $this->assertEquals([
@@ -421,16 +157,12 @@ public function testGetTestClasses() {
    */
   public function testGetTestClassesWithSelectedTypes() {
     $this->setupVfsWithTestClasses();
-    $class_loader = $this->prophesize(ClassLoader::class);
-    $module_handler = $this->prophesize(ModuleHandlerInterface::class);
-
-    $test_discovery = new TestTestDiscovery('vfs://drupal', $class_loader->reveal(), $module_handler->reveal());
-
     $extensions = [
       'test_module' => new Extension('vfs://drupal', 'module', 'modules/test_module/test_module.info.yml'),
       'test_profile_module' => new Extension('vfs://drupal', 'profile', 'profiles/test_profile/modules/test_profile_module/test_profile_module.info.yml'),
     ];
-    $test_discovery->setExtensions($extensions);
+    $test_discovery = $this->getTestDiscoveryMock('vfs://drupal', $extensions);
+
     $result = $test_discovery->getTestClasses(NULL, ['PHPUnit-Kernel']);
     $this->assertCount(4, $result);
     $this->assertEquals([
@@ -495,80 +227,4 @@ public function testGetTestsInProfiles() {
     $this->assertEquals($expected, $result);
   }
 
-  /**
-   * @covers ::getPhpunitTestSuite
-   * @dataProvider providerTestGetPhpunitTestSuite
-   */
-  public function testGetPhpunitTestSuite($classname, $expected) {
-    $this->assertEquals($expected, TestDiscovery::getPhpunitTestSuite($classname));
-  }
-
-  public function providerTestGetPhpunitTestSuite() {
-    $data = [];
-    $data['simpletest-webtest'] = ['\Drupal\rest\Tests\NodeTest', FALSE];
-    $data['simpletest-kerneltest'] = ['\Drupal\hal\Tests\FileNormalizeTest', FALSE];
-    $data['module-unittest'] = [static::class, 'Unit'];
-    $data['module-kerneltest'] = ['\Drupal\KernelTests\Core\Theme\TwigMarkupInterfaceTest', 'Kernel'];
-    $data['module-functionaltest'] = ['\Drupal\FunctionalTests\BrowserTestBaseTest', 'Functional'];
-    $data['module-functionaljavascripttest'] = ['\Drupal\Tests\toolbar\FunctionalJavascript\ToolbarIntegrationTest', 'FunctionalJavascript'];
-    $data['core-unittest'] = ['\Drupal\Tests\ComposerIntegrationTest', 'Unit'];
-    $data['core-unittest2'] = ['Drupal\Tests\Core\DrupalTest', 'Unit'];
-    $data['core-unittest3'] = ['Drupal\Tests\Scripts\TestSiteApplicationTest', 'Unit'];
-    $data['core-kerneltest'] = ['\Drupal\KernelTests\KernelTestBaseTest', 'Kernel'];
-    $data['core-functionaltest'] = ['\Drupal\FunctionalTests\ExampleTest', 'Functional'];
-    $data['core-functionaljavascripttest'] = ['\Drupal\FunctionalJavascriptTests\ExampleTest', 'FunctionalJavascript'];
-
-    return $data;
-  }
-
-  /**
-   * Ensure that classes are not reflected when the docblock is empty.
-   *
-   * @covers ::getTestInfo
-   */
-  public function testGetTestInfoEmptyDocblock() {
-    // If getTestInfo() performed reflection, it won't be able to find the
-    // class we asked it to analyze, so it will throw a ReflectionException.
-    // We want to make sure it didn't do that, because we already did some
-    // analysis and already have an empty docblock. getTestInfo() will throw
-    // MissingGroupException because the annotation is empty.
-    $this->expectException(MissingGroupException::class);
-    TestDiscovery::getTestInfo('Drupal\Tests\simpletest\ThisTestDoesNotExistTest', '');
-  }
-
-  /**
-   * Ensure TestDiscovery::scanDirectory() ignores certain abstract file types.
-   *
-   * @covers ::scanDirectory
-   */
-  public function testScanDirectoryNoAbstract() {
-    $this->setupVfsWithTestClasses();
-    $files = TestDiscovery::scanDirectory('Drupal\\Tests\\test_module\\Kernel\\', vfsStream::url('drupal/modules/test_module/tests/src/Kernel'));
-    $this->assertNotEmpty($files);
-    $this->assertArrayNotHasKey('Drupal\Tests\test_module\Kernel\KernelExampleTestBase', $files);
-    $this->assertArrayNotHasKey('Drupal\Tests\test_module\Kernel\KernelExampleTrait', $files);
-    $this->assertArrayNotHasKey('Drupal\Tests\test_module\Kernel\KernelExampleInterface', $files);
-    $this->assertArrayHasKey('Drupal\Tests\test_module\Kernel\KernelExampleTest3', $files);
-  }
-
-}
-
-class TestTestDiscovery extends TestDiscovery {
-
-  /**
-   * @var \Drupal\Core\Extension\Extension[]
-   */
-  protected $extensions = [];
-
-  public function setExtensions(array $extensions) {
-    $this->extensions = $extensions;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function getExtensions() {
-    return $this->extensions;
-  }
-
 }
diff --git a/core/scripts/run-tests.sh b/core/scripts/run-tests.sh
index 9b39ae46948471495332dddc15f0ba93702e183c..7b2a0088b6f79f7da2e9d6ac16719c2ae4fe1eca 100755
--- a/core/scripts/run-tests.sh
+++ b/core/scripts/run-tests.sh
@@ -18,7 +18,7 @@
 use Drupal\Core\Test\TestRunnerKernel;
 use Drupal\simpletest\Form\SimpletestResultsForm;
 use Drupal\simpletest\TestBase;
-use Drupal\simpletest\TestDiscovery;
+use Drupal\Core\Test\TestDiscovery;
 use PHPUnit\Framework\TestCase;
 use PHPUnit\Runner\Version;
 use Symfony\Component\HttpFoundation\Request;
@@ -65,6 +65,8 @@
   echo "\nAvailable test groups & classes\n";
   echo "-------------------------------\n\n";
   try {
+    // @todo Use \Drupal\Core\Test\TestDiscovery when we no longer need BC for
+    //   hook_simpletest_alter().
     $groups = \Drupal::service('test_discovery')->getTestClasses($args['module']);
   }
   catch (Exception $e) {
@@ -95,6 +97,8 @@
   // List all files which could be run as tests.
   $test_discovery = NULL;
   try {
+    // @todo Use \Drupal\Core\Test\TestDiscovery when we no longer need BC for
+    //   hook_simpletest_alter().
     $test_discovery = \Drupal::service('test_discovery');
   }
   catch (Exception $e) {
@@ -995,6 +999,8 @@ function simpletest_script_cleanup($test_id, $test_class, $exitcode) {
 function simpletest_script_get_test_list() {
   global $args;
 
+  // @todo Use \Drupal\Core\Test\TestDiscovery when we no longer need BC for
+  //   hook_simpletest_alter().
   /** $test_discovery \Drupal\simpletest\TestDiscovery */
   $test_discovery = \Drupal::service('test_discovery');
   $types_processed = empty($args['types']);
diff --git a/core/tests/Drupal/Tests/Core/Test/TestDiscoveryTest.php b/core/tests/Drupal/Tests/Core/Test/TestDiscoveryTest.php
new file mode 100644
index 0000000000000000000000000000000000000000..65266c9d8af8dde2a88260fd7e3f6a49b9b76365
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Test/TestDiscoveryTest.php
@@ -0,0 +1,565 @@
+<?php
+
+namespace Drupal\Tests\Core\Test;
+
+use Composer\Autoload\ClassLoader;
+use Drupal\Core\DependencyInjection\Container;
+use Drupal\Core\DrupalKernel;
+use Drupal\Core\Extension\Extension;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Test\Exception\MissingGroupException;
+use Drupal\Core\Test\TestDiscovery;
+use Drupal\Tests\UnitTestCase;
+use org\bovigo\vfs\vfsStream;
+
+/**
+ * @coversDefaultClass \Drupal\Core\Test\TestDiscovery
+ * @group Test
+ */
+class TestDiscoveryTest extends UnitTestCase {
+
+  /**
+   * @covers ::getTestInfo
+   * @dataProvider infoParserProvider
+   */
+  public function testTestInfoParser($expected, $classname, $doc_comment = NULL) {
+    $info = TestDiscovery::getTestInfo($classname, $doc_comment);
+    $this->assertEquals($expected, $info);
+  }
+
+  public function infoParserProvider() {
+    // A module provided unit test.
+    $tests[] = [
+      // Expected result.
+      [
+        'name' => static::class,
+        'group' => 'Test',
+        'groups' => ['Test'],
+        'description' => 'Tests \Drupal\Core\Test\TestDiscovery.',
+        'type' => 'PHPUnit-Unit',
+      ],
+      // Classname.
+      static::class,
+    ];
+
+    // A core unit test.
+    $tests[] = [
+      // Expected result.
+      [
+        'name' => 'Drupal\Tests\Core\DrupalTest',
+        'group' => 'DrupalTest',
+        'groups' => ['DrupalTest'],
+        'description' => 'Tests \Drupal.',
+        'type' => 'PHPUnit-Unit',
+      ],
+      // Classname.
+      'Drupal\Tests\Core\DrupalTest',
+    ];
+
+    // Functional PHPUnit test.
+    $tests[] = [
+      // Expected result.
+      [
+        'name' => 'Drupal\FunctionalTests\BrowserTestBaseTest',
+        'group' => 'browsertestbase',
+        'groups' => ['browsertestbase'],
+        'description' => 'Tests BrowserTestBase functionality.',
+        'type' => 'PHPUnit-Functional',
+      ],
+      // Classname.
+      'Drupal\FunctionalTests\BrowserTestBaseTest',
+    ];
+
+    // kernel PHPUnit test.
+    $tests['phpunit-kernel'] = [
+      // Expected result.
+      [
+        'name' => '\Drupal\Tests\file\Kernel\FileItemValidationTest',
+        'group' => 'file',
+        'groups' => ['file'],
+        'description' => 'Tests that files referenced in file and image fields are always validated.',
+        'type' => 'PHPUnit-Kernel',
+      ],
+      // Classname.
+      '\Drupal\Tests\file\Kernel\FileItemValidationTest',
+    ];
+
+    // Simpletest classes can not be autoloaded in a PHPUnit test, therefore
+    // provide a docblock.
+    $tests[] = [
+      // Expected result.
+      [
+        'name' => 'Drupal\simpletest\Tests\ExampleSimpleTest',
+        'group' => 'simpletest',
+        'groups' => ['simpletest'],
+        'description' => 'Tests the Simpletest UI internal browser.',
+        'type' => 'Simpletest',
+      ],
+      // Classname.
+      'Drupal\simpletest\Tests\ExampleSimpleTest',
+      // Doc block.
+      "/**
+ * Tests the Simpletest UI internal browser.
+ *
+ * @group simpletest
+ */
+ ",
+    ];
+
+    // Test with a different amount of leading spaces.
+    $tests[] = [
+      // Expected result.
+      [
+        'name' => 'Drupal\simpletest\Tests\ExampleSimpleTest',
+        'group' => 'simpletest',
+        'groups' => ['simpletest'],
+        'description' => 'Tests the Simpletest UI internal browser.',
+        'type' => 'Simpletest',
+      ],
+      // Classname.
+      'Drupal\simpletest\Tests\ExampleSimpleTest',
+      // Doc block.
+      "/**
+   * Tests the Simpletest UI internal browser.
+   *
+   * @group simpletest
+   */
+   */
+ ",
+    ];
+
+    // Make sure that a "* @" inside a string does not get parsed as an
+    // annotation.
+    $tests[] = [
+      // Expected result.
+      [
+        'name' => 'Drupal\simpletest\Tests\ExampleSimpleTest',
+        'group' => 'simpletest',
+        'groups' => ['simpletest'],
+        'description' => 'Tests the Simpletest UI internal browser. * @',
+        'type' => 'Simpletest',
+      ],
+      // Classname.
+      'Drupal\simpletest\Tests\ExampleSimpleTest',
+      // Doc block.
+      "/**
+   * Tests the Simpletest UI internal browser. * @
+   *
+   * @group simpletest
+   */
+ ",
+    ];
+
+    // Multiple @group annotations.
+    $tests[] = [
+      // Expected result.
+      [
+        'name' => 'Drupal\simpletest\Tests\ExampleSimpleTest',
+        'group' => 'Test',
+        'groups' => ['Test', 'simpletest'],
+        'description' => 'Tests the Simpletest UI internal browser.',
+        'type' => 'Simpletest',
+      ],
+      // Classname.
+      'Drupal\simpletest\Tests\ExampleSimpleTest',
+      // Doc block.
+      "/**
+ * Tests the Simpletest UI internal browser.
+ *
+ * @group Test
+ * @group simpletest
+ */
+ ",
+    ];
+
+    // A great number of @group annotations.
+    $tests['many-group-annotations'] = [
+      // Expected result.
+      [
+        'name' => 'Drupal\simpletest\Tests\ExampleSimpleTest',
+        'group' => 'Test',
+        'groups' => ['Test', 'simpletest', 'another', 'more', 'many', 'enough', 'whoa'],
+        'description' => 'Tests the Simpletest UI internal browser.',
+        'type' => 'Simpletest',
+      ],
+      // Classname.
+      'Drupal\simpletest\Tests\ExampleSimpleTest',
+      // Doc block.
+      "/**
+ * Tests the Simpletest UI internal browser.
+ *
+ * @group Test
+ * @group simpletest
+ * @group another
+ * @group more
+ * @group many
+ * @group enough
+ * @group whoa
+ */
+ ",
+    ];
+
+    // @dependencies annotation.
+    $tests[] = [
+      // Expected result.
+      [
+        'name' => 'Drupal\simpletest\Tests\ExampleSimpleTest',
+        'description' => 'Tests the Simpletest UI internal browser.',
+        'type' => 'Simpletest',
+        'requires' => ['module' => ['test']],
+        'group' => 'simpletest',
+        'groups' => ['simpletest'],
+      ],
+      // Classname.
+      'Drupal\simpletest\Tests\ExampleSimpleTest',
+      // Doc block.
+      "/**
+ * Tests the Simpletest UI internal browser.
+ *
+ * @dependencies test
+ * @group simpletest
+ */
+ ",
+    ];
+
+    // Multiple @dependencies annotation.
+    $tests[] = [
+      // Expected result.
+      [
+        'name' => 'Drupal\simpletest\Tests\ExampleSimpleTest',
+        'description' => 'Tests the Simpletest UI internal browser.',
+        'type' => 'Simpletest',
+        'requires' => ['module' => ['test', 'test1', 'test2']],
+        'group' => 'simpletest',
+        'groups' => ['simpletest'],
+      ],
+      // Classname.
+      'Drupal\simpletest\Tests\ExampleSimpleTest',
+      // Doc block.
+      "/**
+ * Tests the Simpletest UI internal browser.
+ *
+ * @dependencies test, test1, test2
+ * @group simpletest
+ */
+ ",
+    ];
+
+    // Multi-line summary line.
+    $tests[] = [
+      // Expected result.
+      [
+        'name' => 'Drupal\simpletest\Tests\ExampleSimpleTest',
+        'description' => 'Tests the Simpletest UI internal browser. And the summary line continues an there is no gap to the annotation.',
+        'type' => 'Simpletest',
+        'group' => 'simpletest',
+        'groups' => ['simpletest'],
+      ],
+      // Classname.
+      'Drupal\simpletest\Tests\ExampleSimpleTest',
+      // Doc block.
+      "/**
+ * Tests the Simpletest UI internal browser. And the summary line continues an
+ * there is no gap to the annotation.
+ *
+ * @group simpletest
+ */
+ ",
+    ];
+    return $tests;
+  }
+
+  /**
+   * @covers ::getTestInfo
+   */
+  public function testTestInfoParserMissingGroup() {
+    $classname = 'Drupal\KernelTests\field\BulkDeleteTest';
+    $doc_comment = <<<EOT
+/**
+ * Bulk delete storages and fields, and clean up afterwards.
+ */
+EOT;
+    $this->expectException(MissingGroupException::class);
+    $this->expectExceptionMessage('Missing @group annotation in Drupal\KernelTests\field\BulkDeleteTest');
+    TestDiscovery::getTestInfo($classname, $doc_comment);
+  }
+
+  /**
+   * @covers ::getTestInfo
+   */
+  public function testTestInfoParserMissingSummary() {
+    $classname = 'Drupal\KernelTests\field\BulkDeleteTest';
+    $doc_comment = <<<EOT
+/**
+ * @group field
+ */
+EOT;
+    $info = TestDiscovery::getTestInfo($classname, $doc_comment);
+    $this->assertEmpty($info['description']);
+  }
+
+  protected function setupVfsWithTestClasses() {
+    vfsStream::setup('drupal');
+
+    $test_file = <<<EOF
+<?php
+
+/**
+ * Test description
+ * @group example
+ */
+class FunctionalExampleTest {}
+EOF;
+
+    $test_profile_info = <<<EOF
+name: Testing
+type: profile
+core: 8.x
+EOF;
+
+    $test_module_info = <<<EOF
+name: Testing
+type: module
+core: 8.x
+EOF;
+
+    vfsStream::create([
+      'modules' => [
+        'test_module' => [
+          'tests' => [
+            'src' => [
+              'Functional' => [
+                'FunctionalExampleTest.php' => $test_file,
+                'FunctionalExampleTest2.php' => str_replace(['FunctionalExampleTest', '@group example'], ['FunctionalExampleTest2', '@group example2'], $test_file),
+              ],
+              'Kernel' => [
+                'KernelExampleTest3.php' => str_replace(['FunctionalExampleTest', '@group example'], ['KernelExampleTest3', "@group example2\n * @group kernel\n"], $test_file),
+                'KernelExampleTestBase.php' => str_replace(['FunctionalExampleTest', '@group example'], ['KernelExampleTestBase', '@group example2'], $test_file),
+                'KernelExampleTrait.php' => str_replace(['FunctionalExampleTest', '@group example'], ['KernelExampleTrait', '@group example2'], $test_file),
+                'KernelExampleInterface.php' => str_replace(['FunctionalExampleTest', '@group example'], ['KernelExampleInterface', '@group example2'], $test_file),
+              ],
+            ],
+          ],
+        ],
+      ],
+      'profiles' => [
+        'test_profile' => [
+          'test_profile.info.yml' => $test_profile_info,
+          'modules' => [
+            'test_profile_module' => [
+              'test_profile_module.info.yml' => $test_module_info,
+              'tests' => [
+                'src' => [
+                  'Kernel' => [
+                    'KernelExampleTest4.php' => str_replace(['FunctionalExampleTest', '@group example'], ['KernelExampleTest4', '@group example3'], $test_file),
+                  ],
+                ],
+              ],
+            ],
+          ],
+        ],
+      ],
+    ]);
+  }
+
+  /**
+   * @covers ::getTestClasses
+   */
+  public function testGetTestClasses() {
+    $this->setupVfsWithTestClasses();
+    $extensions = [
+      'test_module' => new Extension('vfs://drupal', 'module', 'modules/test_module/test_module.info.yml'),
+    ];
+    $test_discovery = $this->getTestDiscoveryMock('vfs://drupal', $extensions);
+
+    $result = $test_discovery->getTestClasses();
+    $this->assertCount(3, $result);
+    $this->assertEquals([
+      'example' => [
+        'Drupal\Tests\test_module\Functional\FunctionalExampleTest' => [
+          'name' => 'Drupal\Tests\test_module\Functional\FunctionalExampleTest',
+          'description' => 'Test description',
+          'group' => 'example',
+          'groups' => ['example'],
+          'type' => 'PHPUnit-Functional',
+        ],
+      ],
+      'example2' => [
+        'Drupal\Tests\test_module\Functional\FunctionalExampleTest2' => [
+          'name' => 'Drupal\Tests\test_module\Functional\FunctionalExampleTest2',
+          'description' => 'Test description',
+          'group' => 'example2',
+          'groups' => ['example2'],
+          'type' => 'PHPUnit-Functional',
+        ],
+        'Drupal\Tests\test_module\Kernel\KernelExampleTest3' => [
+          'name' => 'Drupal\Tests\test_module\Kernel\KernelExampleTest3',
+          'description' => 'Test description',
+          'group' => 'example2',
+          'groups' => ['example2', 'kernel'],
+          'type' => 'PHPUnit-Kernel',
+        ],
+      ],
+      'kernel' => [
+        'Drupal\Tests\test_module\Kernel\KernelExampleTest3' => [
+          'name' => 'Drupal\Tests\test_module\Kernel\KernelExampleTest3',
+          'description' => 'Test description',
+          'group' => 'example2',
+          'groups' => ['example2', 'kernel'],
+          'type' => 'PHPUnit-Kernel',
+        ],
+      ],
+    ], $result);
+  }
+
+  /**
+   * Mock a TestDiscovery object to return specific extension values.
+   */
+  protected function getTestDiscoveryMock($app_root, $extensions) {
+    $class_loader = $this->prophesize(ClassLoader::class);
+    $module_handler = $this->prophesize(ModuleHandlerInterface::class);
+
+    $test_discovery = $this->getMockBuilder(TestDiscovery::class)
+      ->setConstructorArgs([$app_root, $class_loader->reveal(), $module_handler->reveal()])
+      ->setMethods(['getExtensions'])
+      ->getMock();
+
+    $test_discovery->expects($this->any())
+      ->method('getExtensions')
+      ->willReturn($extensions);
+
+    return $test_discovery;
+  }
+
+  /**
+   * @covers ::getTestClasses
+   */
+  public function testGetTestClassesWithSelectedTypes() {
+    $this->setupVfsWithTestClasses();
+    $extensions = [
+      'test_module' => new Extension('vfs://drupal', 'module', 'modules/test_module/test_module.info.yml'),
+      'test_profile_module' => new Extension('vfs://drupal', 'profile', 'profiles/test_profile/modules/test_profile_module/test_profile_module.info.yml'),
+    ];
+    $test_discovery = $this->getTestDiscoveryMock('vfs://drupal', $extensions);
+
+    $result = $test_discovery->getTestClasses(NULL, ['PHPUnit-Kernel']);
+    $this->assertCount(4, $result);
+    $this->assertEquals([
+      'example' => [],
+      'example2' => [
+        'Drupal\Tests\test_module\Kernel\KernelExampleTest3' => [
+          'name' => 'Drupal\Tests\test_module\Kernel\KernelExampleTest3',
+          'description' => 'Test description',
+          'group' => 'example2',
+          'groups' => ['example2', 'kernel'],
+          'type' => 'PHPUnit-Kernel',
+        ],
+      ],
+      'kernel' => [
+        'Drupal\Tests\test_module\Kernel\KernelExampleTest3' => [
+          'name' => 'Drupal\Tests\test_module\Kernel\KernelExampleTest3',
+          'description' => 'Test description',
+          'group' => 'example2',
+          'groups' => ['example2', 'kernel'],
+          'type' => 'PHPUnit-Kernel',
+        ],
+      ],
+      'example3' => [
+        'Drupal\Tests\test_profile_module\Kernel\KernelExampleTest4' => [
+          'name' => 'Drupal\Tests\test_profile_module\Kernel\KernelExampleTest4',
+          'description' => 'Test description',
+          'group' => 'example3',
+          'groups' => ['example3'],
+          'type' => 'PHPUnit-Kernel',
+        ],
+      ],
+    ], $result);
+  }
+
+  /**
+   * @covers ::getTestClasses
+   */
+  public function testGetTestsInProfiles() {
+    $this->setupVfsWithTestClasses();
+    $class_loader = $this->prophesize(ClassLoader::class);
+    $module_handler = $this->prophesize(ModuleHandlerInterface::class);
+
+    $container = new Container();
+    $container->set('kernel', new DrupalKernel('prod', new ClassLoader()));
+    $container->set('site.path', 'sites/default');
+    \Drupal::setContainer($container);
+
+    $test_discovery = new TestDiscovery('vfs://drupal', $class_loader->reveal(), $module_handler->reveal());
+
+    $result = $test_discovery->getTestClasses(NULL, ['PHPUnit-Kernel']);
+    $expected = [
+      'example3' => [
+        'Drupal\Tests\test_profile_module\Kernel\KernelExampleTest4' => [
+          'name' => 'Drupal\Tests\test_profile_module\Kernel\KernelExampleTest4',
+          'description' => 'Test description',
+          'group' => 'example3',
+          'groups' => ['example3'],
+          'type' => 'PHPUnit-Kernel',
+        ],
+      ],
+    ];
+    $this->assertEquals($expected, $result);
+  }
+
+  /**
+   * @covers ::getPhpunitTestSuite
+   * @dataProvider providerTestGetPhpunitTestSuite
+   */
+  public function testGetPhpunitTestSuite($classname, $expected) {
+    $this->assertEquals($expected, TestDiscovery::getPhpunitTestSuite($classname));
+  }
+
+  public function providerTestGetPhpunitTestSuite() {
+    $data = [];
+    $data['simpletest-webtest'] = ['\Drupal\rest\Tests\NodeTest', FALSE];
+    $data['simpletest-kerneltest'] = ['\Drupal\hal\Tests\FileNormalizeTest', FALSE];
+    $data['module-unittest'] = [static::class, 'Unit'];
+    $data['module-kerneltest'] = ['\Drupal\KernelTests\Core\Theme\TwigMarkupInterfaceTest', 'Kernel'];
+    $data['module-functionaltest'] = ['\Drupal\FunctionalTests\BrowserTestBaseTest', 'Functional'];
+    $data['module-functionaljavascripttest'] = ['\Drupal\Tests\toolbar\FunctionalJavascript\ToolbarIntegrationTest', 'FunctionalJavascript'];
+    $data['core-unittest'] = ['\Drupal\Tests\ComposerIntegrationTest', 'Unit'];
+    $data['core-unittest2'] = ['Drupal\Tests\Core\DrupalTest', 'Unit'];
+    $data['core-unittest3'] = ['Drupal\Tests\Scripts\TestSiteApplicationTest', 'Unit'];
+    $data['core-kerneltest'] = ['\Drupal\KernelTests\KernelTestBaseTest', 'Kernel'];
+    $data['core-functionaltest'] = ['\Drupal\FunctionalTests\ExampleTest', 'Functional'];
+    $data['core-functionaljavascripttest'] = ['\Drupal\FunctionalJavascriptTests\ExampleTest', 'FunctionalJavascript'];
+
+    return $data;
+  }
+
+  /**
+   * Ensure that classes are not reflected when the docblock is empty.
+   *
+   * @covers ::getTestInfo
+   */
+  public function testGetTestInfoEmptyDocblock() {
+    // If getTestInfo() performed reflection, it won't be able to find the
+    // class we asked it to analyze, so it will throw a ReflectionException.
+    // We want to make sure it didn't do that, because we already did some
+    // analysis and already have an empty docblock. getTestInfo() will throw
+    // MissingGroupException because the annotation is empty.
+    $this->expectException(MissingGroupException::class);
+    TestDiscovery::getTestInfo('Drupal\Tests\simpletest\ThisTestDoesNotExistTest', '');
+  }
+
+  /**
+   * Ensure TestDiscovery::scanDirectory() ignores certain abstract file types.
+   *
+   * @covers ::scanDirectory
+   */
+  public function testScanDirectoryNoAbstract() {
+    $this->setupVfsWithTestClasses();
+    $files = TestDiscovery::scanDirectory('Drupal\\Tests\\test_module\\Kernel\\', vfsStream::url('drupal/modules/test_module/tests/src/Kernel'));
+    $this->assertNotEmpty($files);
+    $this->assertArrayNotHasKey('Drupal\Tests\test_module\Kernel\KernelExampleTestBase', $files);
+    $this->assertArrayNotHasKey('Drupal\Tests\test_module\Kernel\KernelExampleTrait', $files);
+    $this->assertArrayNotHasKey('Drupal\Tests\test_module\Kernel\KernelExampleInterface', $files);
+    $this->assertArrayHasKey('Drupal\Tests\test_module\Kernel\KernelExampleTest3', $files);
+  }
+
+}
diff --git a/core/tests/TestSuites/TestSuiteBase.php b/core/tests/TestSuites/TestSuiteBase.php
index 41ed61622b779da4006ec3cf758522519688a1ca..06cfc537e967c6e5e47233d8a13b99004e92d574 100644
--- a/core/tests/TestSuites/TestSuiteBase.php
+++ b/core/tests/TestSuites/TestSuiteBase.php
@@ -2,7 +2,7 @@
 
 namespace Drupal\Tests\TestSuites;
 
-use Drupal\simpletest\TestDiscovery;
+use Drupal\Core\Test\TestDiscovery;
 use PHPUnit\Framework\TestSuite;
 
 /**