diff --git a/core/lib/Drupal/Component/Plugin/Discovery/ProcessDecorator.php b/core/lib/Drupal/Component/Plugin/Discovery/ProcessDecorator.php
deleted file mode 100644
index 1a881230d87..00000000000
--- a/core/lib/Drupal/Component/Plugin/Discovery/ProcessDecorator.php
+++ /dev/null
@@ -1,70 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\Component\Plugin\Discovery\ProcessDecorator.
- */
-
-namespace Drupal\Component\Plugin\Discovery;
-
-/**
- * Allows custom processing of the discovered definition.
- *
- * Example use cases include adding in default values for a definition, or
- * providing a backwards compatibility layer for renamed definition properties.
- */
-class ProcessDecorator implements DiscoveryInterface {
-
-  use DiscoveryTrait;
-
-  /**
-   * The Discovery object being decorated.
-   *
-   * @var \Drupal\Component\Plugin\Discovery\DiscoveryInterface
-   */
-  protected $decorated;
-
-  /**
-   * The processor callback to run on each discovered definition.
-   *
-   * @var callable
-   */
-   protected $processCallback;
-
-  /**
-   * Constructs a \Drupal\Component\Plugin\Discovery\ProcessDecorator object.
-   *
-   * @param \Drupal\Component\Plugin\Discovery\DiscoveryInterface $decorated
-   *   The discovery object that is being decorated.
-   * @param callable $process_callback
-   *   The processor callback to run on each discovered definition. The
-   *   callback will be called with the following arguments:
-   *   - array $definition: the discovered definition, that the callback
-   *     should accept by reference and modify in place.
-   *   - string $plugin_id: the corresponding plugin_id.
-   */
-  public function __construct(DiscoveryInterface $decorated, $process_callback) {
-    $this->decorated = $decorated;
-    $this->processCallback = $process_callback;
-  }
-
-  /**
-   * Implements \Drupal\Component\Plugin\Discovery\DicoveryInterface::getDefinitions().
-   */
-  public function getDefinitions() {
-    $definitions = $this->decorated->getDefinitions();
-    foreach ($definitions as $plugin_id => &$definition) {
-      call_user_func_array($this->processCallback, array(&$definition, $plugin_id));
-    }
-    // Allow process callbacks to unset definitions.
-    return array_filter($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);
-  }
-
-}
diff --git a/core/lib/Drupal/Component/Plugin/PluginManagerBase.php b/core/lib/Drupal/Component/Plugin/PluginManagerBase.php
index f3a19b04420..201b2070ef5 100644
--- a/core/lib/Drupal/Component/Plugin/PluginManagerBase.php
+++ b/core/lib/Drupal/Component/Plugin/PluginManagerBase.php
@@ -7,13 +7,12 @@
 
 namespace Drupal\Component\Plugin;
 
-use Drupal\Component\Plugin\Discovery\CachedDiscoveryInterface;
 use Drupal\Component\Plugin\Discovery\DiscoveryTrait;
 
 /**
  * Base class for plugin managers.
  */
-abstract class PluginManagerBase implements PluginManagerInterface, CachedDiscoveryInterface {
+abstract class PluginManagerBase implements PluginManagerInterface {
 
   use DiscoveryTrait;
 
@@ -52,15 +51,6 @@ abstract class PluginManagerBase implements PluginManagerInterface, CachedDiscov
     return $this->discovery->getDefinitions();
   }
 
-  /**
-   * {@inheritdoc}
-   */
-  public function clearCachedDefinitions() {
-    if ($this->discovery instanceof CachedDiscoveryInterface) {
-      $this->discovery->clearCachedDefinitions();
-    }
-  }
-
   /**
    * {@inheritdoc}
    */
diff --git a/core/lib/Drupal/Core/Menu/LocalActionManager.php b/core/lib/Drupal/Core/Menu/LocalActionManager.php
index fc252b1d39e..456b749ebcf 100644
--- a/core/lib/Drupal/Core/Menu/LocalActionManager.php
+++ b/core/lib/Drupal/Core/Menu/LocalActionManager.php
@@ -10,11 +10,8 @@ namespace Drupal\Core\Menu;
 use Drupal\Core\Access\AccessManager;
 use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\Extension\ModuleHandlerInterface;
-use Drupal\Core\Language\LanguageManager;
 use Drupal\Core\Language\LanguageManagerInterface;
-use Drupal\Core\Menu\LocalActionInterface;
 use Drupal\Core\Plugin\DefaultPluginManager;
-use Drupal\Component\Plugin\Discovery\ProcessDecorator;
 use Drupal\Core\Plugin\Discovery\ContainerDerivativeDiscoveryDecorator;
 use Drupal\Core\Plugin\Discovery\YamlDiscovery;
 use Drupal\Core\Plugin\Factory\ContainerFactory;
diff --git a/core/lib/Drupal/Core/Plugin/Discovery/AlterDecorator.php b/core/lib/Drupal/Core/Plugin/Discovery/AlterDecorator.php
deleted file mode 100644
index 0a7cf861854..00000000000
--- a/core/lib/Drupal/Core/Plugin/Discovery/AlterDecorator.php
+++ /dev/null
@@ -1,64 +0,0 @@
-<?php
-
-/**
- * @file
- * Definition of Drupal\Core\Plugin\Discovery\AlterDiscoveryDecorator.
- */
-
-namespace Drupal\Core\Plugin\Discovery;
-
-use Drupal\Component\Plugin\Discovery\DiscoveryInterface;
-use Drupal\Component\Plugin\Discovery\DiscoveryTrait;
-
-/**
- * Enables altering of discovered plugin definitions.
- */
-class AlterDecorator implements DiscoveryInterface {
-
-  use DiscoveryTrait;
-
-  /**
-   * 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 used by this discovery instance.
-   */
-  public function __construct(DiscoveryInterface $decorated, $hook) {
-    $this->decorated = $decorated;
-    $this->hook = $hook;
-  }
-
-  /**
-   * Implements Drupal\Component\Plugin\Discovery\DiscoveryInterface::getDefinitions().
-   */
-  public function getDefinitions() {
-    $definitions = $this->decorated->getDefinitions();
-    \Drupal::moduleHandler()->alter($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);
-  }
-}
diff --git a/core/lib/Drupal/Core/Plugin/Discovery/CacheDecorator.php b/core/lib/Drupal/Core/Plugin/Discovery/CacheDecorator.php
deleted file mode 100644
index 9fd21460515..00000000000
--- a/core/lib/Drupal/Core/Plugin/Discovery/CacheDecorator.php
+++ /dev/null
@@ -1,157 +0,0 @@
-<?php
-
-/**
- * @file
- * Definition of Drupal\Core\Plugin\Discovery\CacheDecorator.
- */
-
-namespace Drupal\Core\Plugin\Discovery;
-
-use Drupal\Component\Plugin\Discovery\CachedDiscoveryInterface;
-use Drupal\Component\Plugin\Discovery\DiscoveryCachedTrait;
-use Drupal\Component\Plugin\Discovery\DiscoveryInterface;
-use Drupal\Core\Cache\Cache;
-
-/**
- * Enables static and persistent caching of discovered plugin definitions.
- */
-class CacheDecorator implements CachedDiscoveryInterface {
-
-  use DiscoveryCachedTrait;
-
-  /**
-   * The cache key used to store the definition list.
-   *
-   * @var string
-   */
-  protected $cacheKey;
-
-  /**
-   * The cache bin used to store the definition list.
-   *
-   * @var string
-   */
-  protected $cacheBin;
-
-  /**
-   * The timestamp indicating when the definition list cache expires.
-   *
-   * @var int
-   */
-  protected $cacheExpire;
-
-  /**
-   * The cache tags associated with the definition list.
-   *
-   * @var array
-   */
-  protected $cacheTags;
-
-  /**
-   * The Discovery object being decorated.
-   *
-   * @var \Drupal\Component\Plugin\Discovery\DiscoveryInterface
-   */
-  protected $decorated;
-
-  /**
-   * Constructs a Drupal\Core\Plugin\Discovery\CacheDecorator 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 $cache_key
-   *   The cache identifier used for storage of the definition list.
-   * @param string $cache_bin
-   *   The cache bin used for storage and retrieval of the definition list.
-   * @param int $cache_expire
-   *   A Unix timestamp indicating that the definition list will be considered
-   *   invalid after this time.
-   * @param array $cache_tags
-   *   The cache tags associated with the definition list.
-   */
-  public function __construct(DiscoveryInterface $decorated, $cache_key, $cache_bin = 'default', $cache_expire = Cache::PERMANENT, array $cache_tags = array()) {
-    $this->decorated = $decorated;
-    $this->cacheKey = $cache_key;
-    $this->cacheBin = $cache_bin;
-    $this->cacheExpire = $cache_expire;
-    $this->cacheTags = $cache_tags;
-  }
-
-  /**
-   * Implements Drupal\Component\Plugin\Discovery\DicoveryInterface::getDefinitions().
-   */
-  public function getDefinitions() {
-    // Optimize for fast access to definitions if they are already in memory.
-    if (isset($this->definitions)) {
-      return $this->definitions;
-    }
-
-    $definitions = $this->getCachedDefinitions();
-    if (!isset($definitions)) {
-      $definitions = $this->decorated->getDefinitions();
-      $this->setCachedDefinitions($definitions);
-    }
-    return $definitions;
-  }
-
-  /**
-   * Returns the cached plugin definitions of the decorated discovery class.
-   *
-   * @return mixed
-   *   On success this will return an array of plugin definitions. On failure
-   *   this should return NULL, indicating to other methods that this has not
-   *   yet been defined. Success with no values should return as an empty array
-   *   and would actually be returned by the getDefinitions() method.
-   */
-  protected function getCachedDefinitions() {
-    if (!isset($this->definitions) && isset($this->cacheKey) && $cache = $this->cache($this->cacheBin)->get($this->cacheKey)) {
-      $this->definitions = $cache->data;
-    }
-    return $this->definitions;
-  }
-
-  /**
-   * Sets a cache of plugin definitions for the decorated discovery class.
-   *
-   * @param array $definitions
-   *   List of definitions to store in cache.
-   */
-  protected function setCachedDefinitions($definitions) {
-    if (isset($this->cacheKey)) {
-      $this->cache($this->cacheBin)->set($this->cacheKey, $definitions, $this->cacheExpire, $this->cacheTags);
-    }
-    $this->definitions = $definitions;
-  }
-
-  /**
-   * Implements \Drupal\Component\Plugin\Discovery\CachedDiscoveryInterface::clearCachedDefinitions().
-   */
-  public function clearCachedDefinitions() {
-    // If there are any cache tags, clear cache based on those.
-    if (!empty($this->cacheTags)) {
-      Cache::deleteTags($this->cacheTags);
-    }
-    // Otherwise, just delete the specified cache key.
-    else if (isset($this->cacheKey)) {
-      $this->cache($this->cacheBin)->delete($this->cacheKey);
-    }
-    $this->definitions = NULL;
-  }
-
-  /**
-   * Passes through all unknown calls onto the decorated object.
-   */
-  public function __call($method, $args) {
-    return call_user_func_array(array($this->decorated, $method), $args);
-  }
-
-  /**
-   * Wraps the \Drupal::cache() method.
-   */
-  protected function cache($bin) {
-    return \Drupal::cache($bin);
-  }
-
-}
diff --git a/core/lib/Drupal/Core/Validation/ConstraintManager.php b/core/lib/Drupal/Core/Validation/ConstraintManager.php
index 94eed864d03..a2b46d30728 100644
--- a/core/lib/Drupal/Core/Validation/ConstraintManager.php
+++ b/core/lib/Drupal/Core/Validation/ConstraintManager.php
@@ -107,7 +107,7 @@ class ConstraintManager extends DefaultPluginManager {
   }
 
   /**
-   * Process definition callback for the ProcessDecorator.
+   * {@inheritdoc}
    */
   public function processDefinition(&$definition, $plugin_id) {
     // Make sure 'type' is set and either an array or FALSE.
diff --git a/core/modules/system/src/Tests/Plugin/AlterDecoratorTest.php b/core/modules/system/src/Tests/Plugin/AlterDecoratorTest.php
deleted file mode 100644
index b6946f354af..00000000000
--- a/core/modules/system/src/Tests/Plugin/AlterDecoratorTest.php
+++ /dev/null
@@ -1,56 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\system\Tests\Plugin\AlterDecoratorTest.
- */
-
-namespace Drupal\system\Tests\Plugin;
-
-use Drupal\plugin_test\Plugin\AlterDecoratorTestPluginManager;
-use Drupal\simpletest\WebTestBase;
-
-/**
- * Tests that the AlterDecorator fires and respects the alter hook.
- *
- * @group Plugin
- */
-class AlterDecoratorTest extends WebTestBase {
-
-  /**
-   * Modules to enable.
-   *
-   * @var array
-   */
-  public static $modules = array('plugin_test');
-
-  /**
-   * Stores a plugin manager which uses the AlterDecorator.
-   *
-   * @var \Drupal\plugin_test\Plugin\AlterDecoratorTestPluginManager;
-   */
-  protected $alterTestPluginManager;
-
-  public function setUp() {
-    parent::setUp();
-
-    // Setup a plugin manager which uses the alter decorator.
-    $this->alterTestPluginManager = new AlterDecoratorTestPluginManager();
-  }
-
-  /**
-   * Tests getDefinitions() and getDefinition() of Drupal\Core\Plugin\Discovery\AlterDecorator.
-   */
-  public function testAlterDecorator() {
-    // Ensure that getDefinitions() fires and changes the actual plugin definitions.
-    $definitions = $this->alterTestPluginManager->getDefinitions();
-    foreach ($definitions as &$definition) {
-      $this->assertTrue($definition['altered']);
-    }
-
-    // Ensure that getDefinitions() fires and changes the actual plugin definition.
-    $definition = $this->alterTestPluginManager->getDefinition('user_login');
-    $this->assertTrue($definition['altered_single']);
-  }
-
-}
diff --git a/core/modules/system/src/Tests/Plugin/CacheDecoratorLanguageTest.php b/core/modules/system/src/Tests/Plugin/CacheDecoratorLanguageTest.php
deleted file mode 100644
index 6be2b6f6256..00000000000
--- a/core/modules/system/src/Tests/Plugin/CacheDecoratorLanguageTest.php
+++ /dev/null
@@ -1,121 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\system\Tests\Plugin\CacheDecoratorLanguageTest.
- */
-
-namespace Drupal\system\Tests\Plugin;
-
-use Drupal\Core\Language\Language;
-use Drupal\plugin_test\Plugin\CachedMockBlockManager;
-use Drupal\simpletest\WebTestBase;
-
-/**
- * Tests that the CacheDecorator stores definitions by language appropriately.
- *
- * @group Plugin
- */
-class CacheDecoratorLanguageTest extends WebTestBase {
-
-  /**
-   * Modules to enable.
-   *
-   * @var array
-   */
-  public static $modules = array('plugin_test', 'locale', 'language');
-
-  public function setUp() {
-    parent::setUp();
-
-    // Populate sample definitions.
-    $this->mockBlockExpectedDefinitions = array(
-      'user_login' => array(
-        'label' => 'User login',
-        'class' => 'Drupal\plugin_test\Plugin\plugin_test\mock_block\MockUserLoginBlock',
-      ),
-      'menu:main_menu' => array(
-        'label' => 'Main menu',
-        'class' => 'Drupal\plugin_test\Plugin\plugin_test\mock_block\MockMenuBlock',
-      ),
-      'menu:navigation' => array(
-        'label' => 'Navigation',
-        'class' => 'Drupal\plugin_test\Plugin\plugin_test\mock_block\MockMenuBlock',
-      ),
-      'layout' => array(
-        'label' => 'Layout',
-        'class' => 'Drupal\plugin_test\Plugin\plugin_test\mock_block\MockLayoutBlock',
-      ),
-      'layout:foo' => array(
-        'label' => 'Layout Foo',
-        'class' => 'Drupal\plugin_test\Plugin\plugin_test\mock_block\MockLayoutBlock',
-      ),
-    );
-
-    // Create two languages: Spanish and German.
-    $this->languages = array('de', 'es');
-    foreach ($this->languages as $langcode) {
-      $language = new Language(array('id' => $langcode));
-      $languages[$langcode] = language_save($language);
-      // Set up translations for each mock block label.
-      $custom_strings = array();
-      foreach ($this->mockBlockExpectedDefinitions as $definition) {
-        $custom_strings[$definition['label']] = $langcode . ' ' . $definition['label'];
-      }
-      $this->addCustomTranslations($langcode, array('' => $custom_strings));
-      $this->rebuildContainer();
-    }
-    // Write test settings.php with new translations.
-    $this->writeCustomTranslations();
-  }
-
-  /**
-   * Check the translations of the cached plugin definitions.
-   */
-  public function testCacheDecoratorLanguage() {
-    $languages = $this->languages;
-    $this->drupalGet('plugin_definition_test');
-    foreach ($this->mockBlockExpectedDefinitions as $definition) {
-      // Find our source text.
-      $this->assertText($definition['label']);
-    }
-    foreach ($languages as $langcode) {
-      $url = $langcode . '/plugin_definition_test';
-      // For each language visit the language specific version of the page again.
-      $this->drupalGet($url);
-      foreach ($this->mockBlockExpectedDefinitions as $definition) {
-        // Find our provided translations.
-        $label = $langcode . ' ' . $definition['label'];
-        $this->assertText($label);
-      }
-    }
-    // Manually check that the expected cache keys are present.
-    $languages[] = 'en';
-    foreach ($languages as $langcode) {
-      $cache = \Drupal::cache()->get('mock_block:' . $langcode);
-      $this->assertEqual($cache->cid, 'mock_block:' . $langcode, format_string('The !cache cache exists.', array('!cache' => 'mock_block:' . $langcode)));
-      $this->assertEqual($cache->expire, 1542646800, format_string('The cache expiration was properly set.'));
-    }
-    // Clear the plugin definitions.
-    $manager = new CachedMockBlockManager();
-    $manager->clearCachedDefinitions();
-    foreach ($languages as $langcode) {
-      $cache = \Drupal::cache()->get('mock_block:' . $langcode);
-      $this->assertFalse($cache, format_string('The !cache cache was properly cleared through the cache::deleteTags() method.', array('!cache' => 'mock_block:' . $langcode)));
-    }
-    // Change the translations for the german language and recheck strings.
-    $custom_strings = array();
-    foreach ($this->mockBlockExpectedDefinitions as $definition) {
-      $custom_strings[$definition['label']] = $definition['label'] . ' de';
-    }
-    $this->addCustomTranslations('de', array('' => $custom_strings));
-    $this->writeCustomTranslations();
-    $this->drupalGet('de/plugin_definition_test');
-    foreach ($this->mockBlockExpectedDefinitions as $definition) {
-      // Find our provided translations.
-      $label = $definition['label'] . ' de';
-      $this->assertText($label);
-    }
-  }
-
-}
diff --git a/core/modules/system/src/Tests/Plugin/CacheDecoratorTest.php b/core/modules/system/src/Tests/Plugin/CacheDecoratorTest.php
deleted file mode 100644
index 99f2d81be13..00000000000
--- a/core/modules/system/src/Tests/Plugin/CacheDecoratorTest.php
+++ /dev/null
@@ -1,131 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\system\Tests\Plugin\CacheDecoratorTest.
- */
-
-namespace Drupal\system\Tests\Plugin;
-
-use Drupal\Core\Cache\MemoryBackend;
-use Drupal\system\Tests\Plugin\Discovery\DiscoveryTestBase;
-use Drupal\Component\Plugin\Discovery\StaticDiscovery;
-use Drupal\Core\Plugin\Discovery\CacheDecorator;
-
-/**
- * Tests the CacheDecorator.
- *
- * @group Plugin
- */
-class CacheDecoratorTest extends DiscoveryTestBase {
-
-  /**
-   * The cache bin.
-   *
-   * @var string
-   */
-  protected $cacheBin = 'test_cacheDecorator';
-
-  /**
-   * The cache key.
-   *
-   * @var string
-   */
-  protected $cacheKey = 'test_cacheDecorator';
-
-  public function setUp() {
-
-    parent::setUp();
-
-    // Use a non-db cache backend, so that we can use DiscoveryTestBase (which
-    // extends UnitTestBase).
-    // @todo switch to injecting the MemoryBackend http://drupal.org/node/1903346
-    \Drupal::getContainer()->set("cache.$this->cacheBin", new MemoryBackend($this->cacheBin));
-
-    // Create discovery objects to test.
-    $this->emptyDiscovery = new StaticDiscovery();
-    $this->emptyDiscovery = new CacheDecorator($this->emptyDiscovery, $this->cacheKey . '_empty', $this->cacheBin);
-
-    $this->discovery = new StaticDiscovery();
-    $this->discovery = new CacheDecorator($this->discovery, $this->cacheKey, $this->cacheBin);
-
-    // Populate sample definitions.
-    $this->expectedDefinitions = array(
-      'apple' => array(
-        'label' => 'Apple',
-        'color' => 'green',
-      ),
-      'cherry' => array(
-        'label' => 'Cherry',
-        'color' => 'red',
-      ),
-      'orange' => array(
-        'label' => 'Orange',
-        'color' => 'orange',
-      ),
-    );
-    foreach ($this->expectedDefinitions as $plugin_id => $definition) {
-      $this->discovery->setDefinition($plugin_id, $definition);
-    }
-  }
-
-  /**
-   * Tests that discovered definitions are properly cached.
-   *
-   * This comes in addition to DiscoveryTestBase::testDiscoveryInterface(),
-   * that test the basic discovery behavior.
-   */
-  public function testCachedDefinitions() {
-    $cache = \Drupal::cache($this->cacheBin);
-
-    // Check that nothing is cached initially.
-    $cached = $cache->get($this->cacheKey);
-    $this->assertIdentical($cached, FALSE, 'Cache is empty.');
-
-    // Get the definitions once, and check that they are present in the cache.
-    $definitions = $this->discovery->getDefinitions();
-    $this->assertIdentical($definitions, $this->expectedDefinitions, 'Definitions are correctly retrieved.');
-    $cached = $cache->get($this->cacheKey);
-    $this->assertIdentical($cached->data, $this->expectedDefinitions, 'Definitions are cached.');
-
-    // Check that the definitions are also cached in memory. Since the
-    // CacheDecorator::definitions property is protected, this is tested "from
-    // the outside" by wiping the cache entry, getting the definitions, and
-    // checking that the cache entry was not regenerated (thus showing that
-    // defintions were not fetched from the decorated discovery).
-    $cache->delete($this->cacheKey);
-    $definitions = $this->discovery->getDefinitions();
-    $cached = $cache->get($this->cacheKey);
-    $this->assertIdentical($cached, FALSE, 'Cache is empty.');
-    $this->assertIdentical($definitions, $this->expectedDefinitions, 'Definitions are cached in memory.');
-  }
-
-  /**
-   * Tests CacheDecorator::clearCachedDefinitions().
-   */
-  public function testClearCachedDefinitions() {
-    $cache = \Drupal::cache($this->cacheBin);
-
-    // Populate the caches by collecting definitions once.
-    $this->discovery->getDefinitions();
-
-    // Add a new definition.
-    $this->expectedDefinitions['banana'] = array(
-      'label' => 'Banana',
-      'color' => 'yellow',
-    );
-    $this->discovery->setDefinition('banana', $this->expectedDefinitions['banana']);
-
-    // Check that the new definition is not found.
-    $definition = $this->discovery->getDefinition('banana', FALSE);
-    $this->assertNull($definition, 'Newly added definition is not found.');
-
-    // Clear cached definitions, and check that the new definition is found.
-    $this->discovery->clearCachedDefinitions();
-    $cached = $cache->get($this->cacheKey);
-    $this->assertIdentical($cached, FALSE, 'Cache is empty.');
-    $definitions = $this->discovery->getDefinitions();
-    $this->assertIdentical($definitions, $this->expectedDefinitions, 'Newly added definition is found.');
-  }
-
-}
diff --git a/core/modules/system/tests/modules/plugin_test/plugin_test.routing.yml b/core/modules/system/tests/modules/plugin_test/plugin_test.routing.yml
deleted file mode 100644
index 228e7697100..00000000000
--- a/core/modules/system/tests/modules/plugin_test/plugin_test.routing.yml
+++ /dev/null
@@ -1,6 +0,0 @@
-plugin_test.definition:
-  path: '/plugin_definition_test'
-  defaults:
-    _content: '\Drupal\plugin_test\Controller\PluginTest::testDefinitions'
-  requirements:
-    _access: 'TRUE'
diff --git a/core/modules/system/tests/modules/plugin_test/src/Controller/PluginTest.php b/core/modules/system/tests/modules/plugin_test/src/Controller/PluginTest.php
deleted file mode 100644
index 5ec7fa41ec1..00000000000
--- a/core/modules/system/tests/modules/plugin_test/src/Controller/PluginTest.php
+++ /dev/null
@@ -1,34 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\plugin_test\Controller\PluginTest.
- */
-
-namespace Drupal\plugin_test\Controller;
-
-use Drupal\plugin_test\Plugin\CachedMockBlockManager;
-
-/**
- * Returns a test page containing plugin labels.
- */
-class PluginTest {
-
-  /**
-   * Prints plugin labels for testing.
-   *
-   * @return array
-   *   A renderable array of plugin labels.
-   */
-  public function testDefinitions() {
-    $manager = new CachedMockBlockManager();
-    $output = array();
-    foreach ($manager->getDefinitions() as $plugin_id => $definition) {
-      $output[$plugin_id] = array(
-        '#markup' => $definition['label'],
-      );
-    }
-    return $output;
-  }
-
-}
diff --git a/core/modules/system/tests/modules/plugin_test/src/Plugin/AlterDecoratorTestPluginManager.php b/core/modules/system/tests/modules/plugin_test/src/Plugin/AlterDecoratorTestPluginManager.php
deleted file mode 100644
index 2168b8beab6..00000000000
--- a/core/modules/system/tests/modules/plugin_test/src/Plugin/AlterDecoratorTestPluginManager.php
+++ /dev/null
@@ -1,20 +0,0 @@
-<?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');
-  }
-}
diff --git a/core/modules/system/tests/modules/plugin_test/src/Plugin/CachedMockBlockManager.php b/core/modules/system/tests/modules/plugin_test/src/Plugin/CachedMockBlockManager.php
deleted file mode 100644
index 4eb2aee3957..00000000000
--- a/core/modules/system/tests/modules/plugin_test/src/Plugin/CachedMockBlockManager.php
+++ /dev/null
@@ -1,29 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains Drupal\plugin_test\Plugin\CachedMockBlockManager.
- */
-
-namespace Drupal\plugin_test\Plugin;
-
-use Drupal\Core\Plugin\Discovery\CacheDecorator;
-
-/**
- * Defines a plugin manager used by Plugin API cache decorator web tests.
- */
-class CachedMockBlockManager extends MockBlockManager {
-
-  /**
-   * Adds a cache decorator to the MockBlockManager's discovery.
-   *
-   * @see \Drupal\plugin_test\Plugin\MockBlockManager::__construct().
-   */
-  public function __construct() {
-    parent::__construct();
-    // The CacheDecorator allows us to cache these plugin definitions for
-    // quicker retrieval. In this case we are generating a cache key by
-    // language.
-    $this->discovery = new CacheDecorator($this->discovery, 'mock_block:' . \Drupal::languageManager()->getCurrentLanguage()->id, 'default',  1542646800, array('plugin_test'));
-  }
-}