dimitriskr
--- a/core/tests/Drupal/Tests/Core/Extension/ThemeExtensionListTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/ThemeExtensionListTest.php
+   *   The theme name to find all its base themes.
+   * @param array $expected
+   *   The expected base themes.
+   *
+   * @dataProvider providerTestDoGetBaseThemes
+   */
+  public function testDoGetBaseThemes(array $themes, $theme, array $expected) {
+    // Mocks and stubs.
+    $module_handler = $this->prophesize(ModuleHandlerInterface::class);
+    $state = new State(new KeyValueMemoryFactory());
+    $config_factory = $this->getConfigFactoryStub([]);
+    $theme_engine_list = $this->prophesize(ThemeEngineExtensionList::class);
+    $theme_listing = new ThemeExtensionList($this->root, 'theme', new NullBackend('test'), new InfoParser($this->root), $module_handler->reveal(), $state, $config_factory, $theme_engine_list->reveal(), 'test');
+
+    $method_to_test = (new \ReflectionObject($theme_listing))->getMethod('doGetBaseThemes');
+    $method_to_test->setAccessible(TRUE);
-    $method_to_test->setAccessible(TRUE);
--- a/core/tests/Drupal/Tests/Core/Extension/ThemeExtensionListTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/ThemeExtensionListTest.php
  }

  /**
-   * Provides test data for testGetBaseThemes.
+   * Tests getting the base themes for a set a defines themes.
+   *
+   * @param array $themes
+   *   An array of available themes, keyed by the theme name.
+   * @param string $theme
+   *   The theme name to find all its base themes.
+   * @param array $expected
+   *   The expected base themes.
+   *
+   * @dataProvider providerTestDoGetBaseThemes
+   */
+  public function testDoGetBaseThemes(array $themes, $theme, array $expected) {
-  public function testDoGetBaseThemes(array $themes, $theme, array $expected) {
+  public function testDoGetBaseThemes(array $themes, $theme, array $expected): void {