Oh hai. How about ALL THE FILES?

core/lib/Drupal/Core/Plugin/Discovery/AlterDecorator.php

+<?php
+
+/**
+ * @file
+ * Definition of Drupal\Core\Plugin\Discovery\AlterDiscoveryDecorator.
+*/
+
+namespace Drupal\Core\Plugin\Discovery;
+
+use Drupal\Component\Plugin\Discovery\DiscoveryInterface;
+
+/**
+ * Enables altering of discovered plugin definitions.
+ */
+class AlterDecorator implements DiscoveryInterface {
+  /**
+   * The name of the alter hook that will be implemented by this discovery instance.
+   *
+   * @var string
+   */
+  protected $hook;
+
+  /**
+   * The Discovery object being decorated.
+   *
+   * @var Drupal\Component\Plugin\Discovery\DiscoveryInterface
+   */
+  protected $decorated;
+
+  /**
+   * Constructs a Drupal\Core\Plugin\Discovery\AlterDecorator object.
+   *
+   * It uses the DiscoveryInterface object it should decorate.
+   *
+   * @param Drupal\Component\Plugin\Discovery\DiscoveryInterface $decorated
+   *   The object implementing DiscoveryInterface that is being decorated.
+   * @param string $hook
+   *   The name of the alter hook that will be implemented by this discovery instance.
+   */
+  public function __construct(DiscoveryInterface $decorated, $hook) {
+    $this->decorated = $decorated;
+    $this->hook = $hook;
+  }
+
+  /**
+   * Implements Drupal\Component\Plugin\Discovery\DiscoveryInterface::getDefinition().
+   */
+  public function getDefinition($plugin_id) {
+    $definitions = $this->getDefinitions();
+    return isset($definitions[$plugin_id]) ? $definitions[$plugin_id] : NULL;
+  }
+
+
+  /**
+   * Implements Drupal\Component\Plugin\Discovery\DiscoveryInterface::getDefinitions().
+   */
+  public function getDefinitions() {
+    $definitions = $this->decorated->getDefinitions();
+    drupal_alter($this->hook, $definitions);
+    return $definitions;
+  }
+
+  /**
+   * Passes through all unknown calls onto the decorated object.
+   */
+  public function __call($method, $args) {
+    return call_user_func_array(array($this->decorated, $method), $args);
+  }
+}

core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/Plugin/AlterDecoratorTestPluginManager.php

+<?php
+
+/**
+ * @file
+ * Definition of Drupal\plugin_test\Plugin\plugin_test\AlterDecoratorTestPluginManager.
+ */
+
+namespace Drupal\plugin_test\Plugin;
+
+use Drupal\Core\Plugin\Discovery\AlterDecorator;
+
+/**
+ * Defines a plugin manager used by AlterDecorator unit tests.
+ */
+class AlterDecoratorTestPluginManager extends TestPluginManager {
+  public function __construct() {
+    parent::__construct();
+    $this->discovery = new AlterDecorator($this->discovery, 'plugin_test');
+  }
+}