diff --git a/core/lib/Drupal/Core/Action/ActionManager.php b/core/lib/Drupal/Core/Action/ActionManager.php
index c987d8c80cf..a3bfeda16f5 100644
--- a/core/lib/Drupal/Core/Action/ActionManager.php
+++ b/core/lib/Drupal/Core/Action/ActionManager.php
@@ -28,7 +28,7 @@ class ActionManager extends PluginManagerBase {
    *   keyed by the corresponding namespace to look for plugin implementations.
    */
   public function __construct(\Traversable $namespaces) {
-    $this->discovery = new AnnotatedClassDiscovery('Action', $namespaces, array(), 'Drupal\Core\Annotation\Action');
+    $this->discovery = new AnnotatedClassDiscovery('Plugin/Action', $namespaces, array(), 'Drupal\Core\Annotation\Action');
     $this->discovery = new AlterDecorator($this->discovery, 'action_info');
 
     $this->factory = new ContainerFactory($this);
diff --git a/core/lib/Drupal/Core/Archiver/ArchiverManager.php b/core/lib/Drupal/Core/Archiver/ArchiverManager.php
index f3bfc966f9b..526dcdc571e 100644
--- a/core/lib/Drupal/Core/Archiver/ArchiverManager.php
+++ b/core/lib/Drupal/Core/Archiver/ArchiverManager.php
@@ -31,7 +31,7 @@ class ArchiverManager extends DefaultPluginManager {
    *   The module handler to invoke the alter hook with.
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, LanguageManager $language_manager, ModuleHandlerInterface $module_handler) {
-    parent::__construct('Archiver', $namespaces);
+    parent::__construct('Plugin/Archiver', $namespaces);
     $this->alterInfo($module_handler, 'archiver_info');
     $this->setCacheBackend($cache_backend, $language_manager, 'archiver_info');
   }
diff --git a/core/lib/Drupal/Core/Condition/ConditionManager.php b/core/lib/Drupal/Core/Condition/ConditionManager.php
index 289774325d9..7f5d6305039 100644
--- a/core/lib/Drupal/Core/Condition/ConditionManager.php
+++ b/core/lib/Drupal/Core/Condition/ConditionManager.php
@@ -39,7 +39,7 @@ class ConditionManager extends DefaultPluginManager implements ExecutableManager
     $annotation_namespaces = array(
       'Drupal\Core\Condition\Annotation' => DRUPAL_ROOT . '/core/lib',
     );
-    parent::__construct('Condition', $namespaces, $annotation_namespaces, 'Drupal\Core\Condition\Annotation\Condition');
+    parent::__construct('Plugin/Condition', $namespaces, $annotation_namespaces, 'Drupal\Core\Condition\Annotation\Condition');
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Entity/EntityManager.php b/core/lib/Drupal/Core/Entity/EntityManager.php
index 4766a91e09e..3312d3a2591 100644
--- a/core/lib/Drupal/Core/Entity/EntityManager.php
+++ b/core/lib/Drupal/Core/Entity/EntityManager.php
@@ -112,7 +112,7 @@ class EntityManager extends PluginManagerBase {
     $this->cache = $cache;
     $this->languageManager = $language_manager;
 
-    $this->discovery = new AnnotatedClassDiscovery('Core/Entity', $namespaces, $annotation_namespaces, 'Drupal\Core\Entity\Annotation\EntityType');
+    $this->discovery = new AnnotatedClassDiscovery('Plugin/Core/Entity', $namespaces, $annotation_namespaces, 'Drupal\Core\Entity\Annotation\EntityType');
     $this->discovery = new InfoHookDecorator($this->discovery, 'entity_info');
     $this->discovery = new AlterDecorator($this->discovery, 'entity_info');
     $this->discovery = new CacheDecorator($this->discovery, 'entity_info:' . $this->languageManager->getLanguage(Language::TYPE_INTERFACE)->id, 'cache', CacheBackendInterface::CACHE_PERMANENT, array('entity_info' => TRUE));
diff --git a/core/lib/Drupal/Core/Entity/Field/FieldTypePluginManager.php b/core/lib/Drupal/Core/Entity/Field/FieldTypePluginManager.php
index 9f9ceb8f5e9..a8c1ef5dd1d 100644
--- a/core/lib/Drupal/Core/Entity/Field/FieldTypePluginManager.php
+++ b/core/lib/Drupal/Core/Entity/Field/FieldTypePluginManager.php
@@ -45,7 +45,7 @@ class FieldTypePluginManager extends DefaultPluginManager {
     $annotation_namespaces = array(
       'Drupal\Core\Entity\Annotation' => DRUPAL_ROOT . '/core/lib',
     );
-    parent::__construct('field/field_type', $namespaces, $annotation_namespaces, 'Drupal\Core\Entity\Annotation\FieldType');
+    parent::__construct('Plugin/field/field_type', $namespaces, $annotation_namespaces, 'Drupal\Core\Entity\Annotation\FieldType');
     $this->alterInfo($module_handler, 'field_info');
     $this->setCacheBackend($cache_backend, $language_manager, 'field_types');
 
diff --git a/core/lib/Drupal/Core/Menu/LocalActionManager.php b/core/lib/Drupal/Core/Menu/LocalActionManager.php
index 7857f7a910a..f4aab248fd3 100644
--- a/core/lib/Drupal/Core/Menu/LocalActionManager.php
+++ b/core/lib/Drupal/Core/Menu/LocalActionManager.php
@@ -59,7 +59,7 @@ class LocalActionManager extends DefaultPluginManager {
    *   The module handler.
    */
   public function __construct(\Traversable $namespaces, ControllerResolverInterface $controller_resolver, Request $request, ModuleHandlerInterface $module_handler) {
-    parent::__construct('Menu\LocalAction', $namespaces, array(), 'Drupal\Core\Annotation\Menu\LocalAction');
+    parent::__construct('Plugin/Menu/LocalAction', $namespaces, array(), 'Drupal\Core\Annotation\Menu\LocalAction');
 
     $this->controllerResolver = $controller_resolver;
     $this->request = $request;
diff --git a/core/lib/Drupal/Core/Menu/LocalTaskManager.php b/core/lib/Drupal/Core/Menu/LocalTaskManager.php
index 12c501cbc85..00837c5f147 100644
--- a/core/lib/Drupal/Core/Menu/LocalTaskManager.php
+++ b/core/lib/Drupal/Core/Menu/LocalTaskManager.php
@@ -66,7 +66,7 @@ class LocalTaskManager extends DefaultPluginManager {
    *   The module handler.u
    */
   public function __construct(\Traversable $namespaces, ControllerResolverInterface $controller_resolver, Request $request, RouteProviderInterface $route_provider, ModuleHandlerInterface $module_handler) {
-    parent::__construct('Menu\LocalTask', $namespaces, array(), 'Drupal\Core\Annotation\Menu\LocalTask');
+    parent::__construct('Plugin/Menu/LocalTask', $namespaces, array(), 'Drupal\Core\Annotation\Menu\LocalTask');
     $this->controllerResolver = $controller_resolver;
     $this->request = $request;
     $this->routeProvider = $route_provider;
diff --git a/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php b/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
index b7b0557a7dc..daddaa75ab3 100644
--- a/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
+++ b/core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
@@ -60,9 +60,10 @@ class DefaultPluginManager extends PluginManagerBase implements PluginManagerInt
   protected $alterHook;
 
   /**
-   * The plugin's subdirectory, for example views/filter.
+   * The subdirectory within a namespace to look for plugins, or FALSE if the
+   * plugins are in the top level of the namespace.
    *
-   * @var string
+   * @var string|bool
    */
   protected $subdir;
 
@@ -83,11 +84,11 @@ class DefaultPluginManager extends PluginManagerBase implements PluginManagerInt
   /**
    * Creates the discovery object.
    *
-   * @param string $subdir
-   *   The plugin's subdirectory, for example views/filter.
+   * @param string|bool $subdir
+   *   The plugin's subdirectory, for example Plugin/views/filter.
    * @param \Traversable $namespaces
    *   An object that implements \Traversable which contains the root paths
-   *   keyed by the corresponding namespace to look for plugin implementations
+   *   keyed by the corresponding namespace to look for plugin implementations.
    * @param array $annotation_namespaces
    *   (optional) The namespaces of classes that can be used as annotations.
    *   Defaults to an empty array.
diff --git a/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php b/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php
index 6695db76db0..63c87ab0776 100644
--- a/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php
+++ b/core/lib/Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php
@@ -15,18 +15,14 @@ use Drupal\Component\Plugin\Discovery\AnnotatedClassDiscovery as ComponentAnnota
 class AnnotatedClassDiscovery extends ComponentAnnotatedClassDiscovery {
 
   /**
-   * The module name that defines the plugin type.
+   * The subdirectory within a namespace to look for plugins.
+   *
+   * If the plugins are in the top level of the namespace and not within a
+   * subdirectory, set this to an empty string.
    *
    * @var string
    */
-  protected $owner;
-
-  /**
-   * The plugin type, for example filter.
-   *
-   * @var string
-   */
-  protected $type;
+  protected $subdir = '';
 
   /**
    * An object containing the namespaces to look for plugin implementations.
@@ -39,11 +35,12 @@ class AnnotatedClassDiscovery extends ComponentAnnotatedClassDiscovery {
    * Constructs an AnnotatedClassDiscovery object.
    *
    * @param string $subdir
-   *   The plugin's subdirectory, for example views/filter.
+   *   Either the plugin's subdirectory, for example 'Plugin/views/filter', or
+   *   empty string if plugins are located at the top level of the namespace.
    * @param \Traversable $root_namespaces
    *   An object that implements \Traversable which contains the root paths
-   *   keyed by the corresponding namespace to look for plugin implementations,
-   *   \Plugin\$subdir will be appended to each namespace.
+   *   keyed by the corresponding namespace to look for plugin implementations.
+   *   If $subdir is not an empty string, it will be appended to each namespace.
    * @param array $annotation_namespaces
    *   (optional) The namespaces of classes that can be used as annotations.
    *   Defaults to an empty array.
@@ -52,7 +49,9 @@ class AnnotatedClassDiscovery extends ComponentAnnotatedClassDiscovery {
    *   Defaults to 'Drupal\Component\Annotation\Plugin'.
    */
   function __construct($subdir, \Traversable $root_namespaces, $annotation_namespaces = array(), $plugin_definition_annotation_name = 'Drupal\Component\Annotation\Plugin') {
-    $this->subdir = str_replace('/', '\\', $subdir);
+    if ($subdir) {
+      $this->subdir = str_replace('/', '\\', $subdir);
+    }
     $this->rootNamespacesIterator = $root_namespaces;
     $annotation_namespaces += array(
       'Drupal\Component\Annotation' => DRUPAL_ROOT . '/core/lib',
@@ -101,7 +100,10 @@ class AnnotatedClassDiscovery extends ComponentAnnotatedClassDiscovery {
   protected function getPluginNamespaces() {
     $plugin_namespaces = array();
     foreach ($this->rootNamespacesIterator as $namespace => $dir) {
-      $plugin_namespaces["$namespace\\Plugin\\{$this->subdir}"] = array($dir);
+      if ($this->subdir) {
+        $namespace .= "\\{$this->subdir}";
+      }
+      $plugin_namespaces[$namespace] = array($dir);
     }
 
     return $plugin_namespaces;
diff --git a/core/lib/Drupal/Core/TypedData/TypedDataManager.php b/core/lib/Drupal/Core/TypedData/TypedDataManager.php
index 5c48fb03508..f217fa5ce5c 100644
--- a/core/lib/Drupal/Core/TypedData/TypedDataManager.php
+++ b/core/lib/Drupal/Core/TypedData/TypedDataManager.php
@@ -52,7 +52,7 @@ class TypedDataManager extends DefaultPluginManager {
     $annotation_namespaces = array(
       'Drupal\Core\TypedData\Annotation' => DRUPAL_ROOT . '/core/lib',
     );
-    parent::__construct('DataType', $namespaces, $annotation_namespaces, 'Drupal\Core\TypedData\Annotation\DataType');
+    parent::__construct('Plugin/DataType', $namespaces, $annotation_namespaces, 'Drupal\Core\TypedData\Annotation\DataType');
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Validation/ConstraintManager.php b/core/lib/Drupal/Core/Validation/ConstraintManager.php
index 12849784b8f..91551bc187c 100644
--- a/core/lib/Drupal/Core/Validation/ConstraintManager.php
+++ b/core/lib/Drupal/Core/Validation/ConstraintManager.php
@@ -47,7 +47,7 @@ class ConstraintManager extends DefaultPluginManager {
    *   The module handler to invoke the alter hook with.
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, LanguageManager $language_manager, ModuleHandlerInterface $module_handler) {
-    parent::__construct('Validation/Constraint', $namespaces);
+    parent::__construct('Plugin/Validation/Constraint', $namespaces);
     $this->discovery = new StaticDiscoveryDecorator($this->discovery, array($this, 'registerDefinitions'));
     $this->alterInfo($module_handler, 'validation_constraint');
     $this->setCacheBackend($cache_backend, $language_manager, 'validation_constraint');
diff --git a/core/modules/aggregator/lib/Drupal/aggregator/Plugin/AggregatorPluginManager.php b/core/modules/aggregator/lib/Drupal/aggregator/Plugin/AggregatorPluginManager.php
index 2aa673159d4..67d8c4dbd67 100644
--- a/core/modules/aggregator/lib/Drupal/aggregator/Plugin/AggregatorPluginManager.php
+++ b/core/modules/aggregator/lib/Drupal/aggregator/Plugin/AggregatorPluginManager.php
@@ -38,7 +38,7 @@ class AggregatorPluginManager extends PluginManagerBase {
       'Drupal\aggregator\Annotation' => DRUPAL_ROOT . '/core/modules/aggregator/lib',
     );
 
-    $this->discovery = new AnnotatedClassDiscovery("aggregator/$type", $namespaces, $annotation_namespaces, $type_annotations[$type]);
+    $this->discovery = new AnnotatedClassDiscovery("Plugin/aggregator/$type", $namespaces, $annotation_namespaces, $type_annotations[$type]);
     $this->discovery = new CacheDecorator($this->discovery, "aggregator_$type:" . language(Language::TYPE_INTERFACE)->id);
     $this->factory = new DefaultFactory($this->discovery);
   }
diff --git a/core/modules/block/lib/Drupal/block/Plugin/Type/BlockManager.php b/core/modules/block/lib/Drupal/block/Plugin/Type/BlockManager.php
index 3963262344f..34daaca738f 100644
--- a/core/modules/block/lib/Drupal/block/Plugin/Type/BlockManager.php
+++ b/core/modules/block/lib/Drupal/block/Plugin/Type/BlockManager.php
@@ -33,7 +33,7 @@ class BlockManager extends DefaultPluginManager {
    *   The module handler to invoke the alter hook with.
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, LanguageManager $language_manager, ModuleHandlerInterface $module_handler) {
-    parent::__construct('Block', $namespaces);
+    parent::__construct('Plugin/Block', $namespaces);
     $this->alterInfo($module_handler, 'block');
     $this->setCacheBackend($cache_backend, $language_manager, 'block_plugins');
   }
diff --git a/core/modules/ckeditor/lib/Drupal/ckeditor/CKEditorPluginManager.php b/core/modules/ckeditor/lib/Drupal/ckeditor/CKEditorPluginManager.php
index 9ed45b83ccc..da2bcfd8a71 100644
--- a/core/modules/ckeditor/lib/Drupal/ckeditor/CKEditorPluginManager.php
+++ b/core/modules/ckeditor/lib/Drupal/ckeditor/CKEditorPluginManager.php
@@ -34,7 +34,7 @@ class CKEditorPluginManager extends DefaultPluginManager {
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, LanguageManager $language_manager, ModuleHandlerInterface $module_handler) {
     $annotation_namespaces = array('Drupal\ckeditor\Annotation' => $namespaces['Drupal\ckeditor']);
-    parent::__construct('CKEditorPlugin', $namespaces, $annotation_namespaces, 'Drupal\ckeditor\Annotation\CKEditorPlugin');
+    parent::__construct('Plugin/CKEditorPlugin', $namespaces, $annotation_namespaces, 'Drupal\ckeditor\Annotation\CKEditorPlugin');
     $this->alterInfo($module_handler, 'ckeditor_plugin_info');
     $this->setCacheBackend($cache_backend, $language_manager, 'ckeditor_plugin');
   }
diff --git a/core/modules/edit/lib/Drupal/edit/Plugin/InPlaceEditorManager.php b/core/modules/edit/lib/Drupal/edit/Plugin/InPlaceEditorManager.php
index f4c544b9a0f..79f8013d372 100644
--- a/core/modules/edit/lib/Drupal/edit/Plugin/InPlaceEditorManager.php
+++ b/core/modules/edit/lib/Drupal/edit/Plugin/InPlaceEditorManager.php
@@ -30,7 +30,7 @@ class InPlaceEditorManager extends PluginManagerBase {
    */
   public function __construct(\Traversable $namespaces) {
     $annotation_namespaces = array('Drupal\edit\Annotation' => $namespaces['Drupal\edit']);
-    $this->discovery = new AnnotatedClassDiscovery('InPlaceEditor', $namespaces, $annotation_namespaces, 'Drupal\edit\Annotation\InPlaceEditor');
+    $this->discovery = new AnnotatedClassDiscovery('Plugin/InPlaceEditor', $namespaces, $annotation_namespaces, 'Drupal\edit\Annotation\InPlaceEditor');
     $this->discovery = new AlterDecorator($this->discovery, 'edit_editor');
     $this->discovery = new CacheDecorator($this->discovery, 'edit:editor');
     $this->factory = new DefaultFactory($this->discovery);
diff --git a/core/modules/editor/lib/Drupal/editor/Plugin/EditorManager.php b/core/modules/editor/lib/Drupal/editor/Plugin/EditorManager.php
index c6126c63fac..9adf2676c9d 100644
--- a/core/modules/editor/lib/Drupal/editor/Plugin/EditorManager.php
+++ b/core/modules/editor/lib/Drupal/editor/Plugin/EditorManager.php
@@ -32,7 +32,7 @@ class EditorManager extends DefaultPluginManager {
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, LanguageManager $language_manager, ModuleHandlerInterface $module_handler) {
     $annotation_namespaces = array('Drupal\editor\Annotation' => $namespaces['Drupal\editor']);
-    parent::__construct('Editor', $namespaces, $annotation_namespaces, 'Drupal\editor\Annotation\Editor');
+    parent::__construct('Plugin/Editor', $namespaces, $annotation_namespaces, 'Drupal\editor\Annotation\Editor');
     $this->alterInfo($module_handler, 'editor_info');
     $this->setCacheBackend($cache_backend, $language_manager, 'editor');
   }
diff --git a/core/modules/entity_reference/lib/Drupal/entity_reference/Plugin/Type/SelectionPluginManager.php b/core/modules/entity_reference/lib/Drupal/entity_reference/Plugin/Type/SelectionPluginManager.php
index c5ecb8ff213..bce6ebfb8ae 100644
--- a/core/modules/entity_reference/lib/Drupal/entity_reference/Plugin/Type/SelectionPluginManager.php
+++ b/core/modules/entity_reference/lib/Drupal/entity_reference/Plugin/Type/SelectionPluginManager.php
@@ -25,11 +25,8 @@ class SelectionPluginManager extends DefaultPluginManager {
    * {@inheritdoc}
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, LanguageManager $language_manager, ModuleHandlerInterface $module_handler) {
-    $this->subdir = 'entity_reference/selection';
-    $annotation_namespaces = array(
-      'Drupal\entity_reference\Annotation' => $namespaces['Drupal\entity_reference']
-    );
-    $this->discovery = new AnnotatedClassDiscovery($this->subdir, $namespaces, $annotation_namespaces, 'Drupal\entity_reference\Annotation\EntityReferenceSelection');
+    $annotation_namespaces = array('Drupal\entity_reference\Annotation' => $namespaces['Drupal\entity_reference']);
+    $this->discovery = new AnnotatedClassDiscovery('Plugin/entity_reference/selection', $namespaces, $annotation_namespaces, 'Drupal\entity_reference\Annotation\EntityReferenceSelection');
 
     // We're not using the parent constructor because we use a different factory
     // method and don't need the derivative discovery decorator.
diff --git a/core/modules/field/lib/Drupal/field/Plugin/Type/Formatter/FormatterPluginManager.php b/core/modules/field/lib/Drupal/field/Plugin/Type/Formatter/FormatterPluginManager.php
index 6eb0c9a73c2..2e19a0f973f 100644
--- a/core/modules/field/lib/Drupal/field/Plugin/Type/Formatter/FormatterPluginManager.php
+++ b/core/modules/field/lib/Drupal/field/Plugin/Type/Formatter/FormatterPluginManager.php
@@ -46,7 +46,7 @@ class FormatterPluginManager extends DefaultPluginManager {
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, ModuleHandlerInterface $module_handler, LanguageManager $language_manager) {
     $annotation_namespaces = array('Drupal\field\Annotation' => $namespaces['Drupal\field']);
 
-    parent::__construct('field/formatter', $namespaces, $annotation_namespaces, 'Drupal\field\Annotation\FieldFormatter');
+    parent::__construct('Plugin/field/formatter', $namespaces, $annotation_namespaces, 'Drupal\field\Annotation\FieldFormatter');
 
     $this->setCacheBackend($cache_backend, $language_manager, 'field_formatter_types');
     $this->alterInfo($module_handler, 'field_formatter_info');
diff --git a/core/modules/field/lib/Drupal/field/Plugin/Type/Widget/WidgetPluginManager.php b/core/modules/field/lib/Drupal/field/Plugin/Type/Widget/WidgetPluginManager.php
index 6ab6eef759b..a5140f9edbc 100644
--- a/core/modules/field/lib/Drupal/field/Plugin/Type/Widget/WidgetPluginManager.php
+++ b/core/modules/field/lib/Drupal/field/Plugin/Type/Widget/WidgetPluginManager.php
@@ -46,7 +46,7 @@ class WidgetPluginManager extends DefaultPluginManager {
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, ModuleHandlerInterface $module_handler, LanguageManager $language_manager) {
     $annotation_namespaces = array('Drupal\field\Annotation' => $namespaces['Drupal\field']);
 
-    parent::__construct('field/widget', $namespaces, $annotation_namespaces, 'Drupal\field\Annotation\FieldWidget');
+    parent::__construct('Plugin/field/widget', $namespaces, $annotation_namespaces, 'Drupal\field\Annotation\FieldWidget');
 
     $this->setCacheBackend($cache_backend, $language_manager, 'field_widget_types');
     $this->alterInfo($module_handler, 'field_widget_info');
diff --git a/core/modules/filter/lib/Drupal/filter/FilterPluginManager.php b/core/modules/filter/lib/Drupal/filter/FilterPluginManager.php
index 881cdf71359..21d34863f9d 100644
--- a/core/modules/filter/lib/Drupal/filter/FilterPluginManager.php
+++ b/core/modules/filter/lib/Drupal/filter/FilterPluginManager.php
@@ -31,7 +31,7 @@ class FilterPluginManager extends PluginManagerBase {
    */
   public function __construct(\Traversable $namespaces) {
     $annotation_namespaces = array('Drupal\filter\Annotation' => $namespaces['Drupal\filter']);
-    $this->discovery = new AnnotatedClassDiscovery('Filter', $namespaces, $annotation_namespaces, 'Drupal\filter\Annotation\Filter');
+    $this->discovery = new AnnotatedClassDiscovery('Plugin/Filter', $namespaces, $annotation_namespaces, 'Drupal\filter\Annotation\Filter');
     $this->discovery = new AlterDecorator($this->discovery, 'filter_info');
     $cache_key = 'filter_plugins:' . language(Language::TYPE_INTERFACE)->id;
     $cache_tags = array('filter_formats' => TRUE);
diff --git a/core/modules/image/lib/Drupal/image/ImageEffectManager.php b/core/modules/image/lib/Drupal/image/ImageEffectManager.php
index 95fc4c15202..64e251e955e 100644
--- a/core/modules/image/lib/Drupal/image/ImageEffectManager.php
+++ b/core/modules/image/lib/Drupal/image/ImageEffectManager.php
@@ -22,7 +22,7 @@ class ImageEffectManager extends DefaultPluginManager {
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, LanguageManager $language_manager, ModuleHandlerInterface $module_handler) {
     $annotation_namespaces = array('Drupal\image\Annotation' => $namespaces['Drupal\image']);
-    parent::__construct('ImageEffect', $namespaces, $annotation_namespaces, 'Drupal\image\Annotation\ImageEffect');
+    parent::__construct('Plugin/ImageEffect', $namespaces, $annotation_namespaces, 'Drupal\image\Annotation\ImageEffect');
 
     $this->alterInfo($module_handler, 'image_effect_info');
     $this->setCacheBackend($cache_backend, $language_manager, 'image_effect');
diff --git a/core/modules/layout/lib/Drupal/layout/Plugin/Type/LayoutManager.php b/core/modules/layout/lib/Drupal/layout/Plugin/Type/LayoutManager.php
index e22623f1ea0..248414efe89 100644
--- a/core/modules/layout/lib/Drupal/layout/Plugin/Type/LayoutManager.php
+++ b/core/modules/layout/lib/Drupal/layout/Plugin/Type/LayoutManager.php
@@ -31,7 +31,7 @@ class LayoutManager extends PluginManagerBase {
    */
   public function __construct(\Traversable $namespaces) {
     // Create layout plugin derivatives from declaratively defined layouts.
-    $this->discovery = new AnnotatedClassDiscovery('Layout', $namespaces);
+    $this->discovery = new AnnotatedClassDiscovery('Plugin/Layout', $namespaces);
     $this->discovery = new DerivativeDiscoveryDecorator($this->discovery);
     $this->discovery = new ProcessDecorator($this->discovery, array($this, 'processDefinition'));
 
diff --git a/core/modules/rest/lib/Drupal/rest/Plugin/Type/ResourcePluginManager.php b/core/modules/rest/lib/Drupal/rest/Plugin/Type/ResourcePluginManager.php
index 6edea1718c2..602063458ca 100644
--- a/core/modules/rest/lib/Drupal/rest/Plugin/Type/ResourcePluginManager.php
+++ b/core/modules/rest/lib/Drupal/rest/Plugin/Type/ResourcePluginManager.php
@@ -28,7 +28,7 @@ class ResourcePluginManager extends PluginManagerBase {
    */
   public function __construct(\Traversable $namespaces) {
     // Create resource plugin derivatives from declaratively defined resources.
-    $this->discovery = new AnnotatedClassDiscovery('rest/resource', $namespaces);
+    $this->discovery = new AnnotatedClassDiscovery('Plugin/rest/resource', $namespaces);
     $this->discovery = new DerivativeDiscoveryDecorator($this->discovery);
     $this->discovery = new AlterDecorator($this->discovery, 'rest_resource');
     $this->discovery = new CacheDecorator($this->discovery, 'rest');
diff --git a/core/modules/system/lib/Drupal/system/Plugin/ImageToolkitManager.php b/core/modules/system/lib/Drupal/system/Plugin/ImageToolkitManager.php
index ddad9c64968..f8e334da96a 100644
--- a/core/modules/system/lib/Drupal/system/Plugin/ImageToolkitManager.php
+++ b/core/modules/system/lib/Drupal/system/Plugin/ImageToolkitManager.php
@@ -28,7 +28,7 @@ class ImageToolkitManager extends DefaultPluginManager {
    *   The language manager.
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, LanguageManager $language_manager) {
-    parent::__construct('ImageToolkit', $namespaces);
+    parent::__construct('Plugin/ImageToolkit', $namespaces);
     $this->setCacheBackend($cache_backend, $language_manager, 'image_toolkit');
   }
 
diff --git a/core/modules/system/lib/Drupal/system/Plugin/Type/PluginUIManager.php b/core/modules/system/lib/Drupal/system/Plugin/Type/PluginUIManager.php
index 0514fe386a6..9328a5c3394 100644
--- a/core/modules/system/lib/Drupal/system/Plugin/Type/PluginUIManager.php
+++ b/core/modules/system/lib/Drupal/system/Plugin/Type/PluginUIManager.php
@@ -29,7 +29,7 @@ class PluginUIManager extends PluginManagerBase {
    *   keyed by the corresponding namespace to look for plugin implementations,
    */
   public function __construct(\Traversable $namespaces) {
-    $this->discovery = new AnnotatedClassDiscovery('PluginUI', $namespaces);
+    $this->discovery = new AnnotatedClassDiscovery('Plugin/PluginUI', $namespaces);
     $this->discovery = new DerivativeDiscoveryDecorator($this->discovery);
     $this->discovery = new AlterDecorator($this->discovery, 'plugin_ui');
     $this->discovery = new CacheDecorator($this->discovery, 'plugin_ui');
diff --git a/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/AnnotatedClassDiscoveryTest.php b/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/AnnotatedClassDiscoveryTest.php
index 5f8ee2d0896..6f43b57b806 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/AnnotatedClassDiscoveryTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/AnnotatedClassDiscoveryTest.php
@@ -58,8 +58,8 @@ class AnnotatedClassDiscoveryTest extends DiscoveryTestBase {
       ),
     );
     $namespaces = new \ArrayObject(array('Drupal\plugin_test' => DRUPAL_ROOT . '/core/modules/system/tests/modules/plugin_test/lib'));
-    $this->discovery = new AnnotatedClassDiscovery('plugin_test/fruit', $namespaces);
-    $this->emptyDiscovery = new AnnotatedClassDiscovery('non_existing_module/non_existing_plugin_type', $namespaces);
+    $this->discovery = new AnnotatedClassDiscovery('Plugin/plugin_test/fruit', $namespaces);
+    $this->emptyDiscovery = new AnnotatedClassDiscovery('Plugin/non_existing_module/non_existing_plugin_type', $namespaces);
   }
 
 }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/CustomAnnotationClassDiscoveryTest.php b/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/CustomAnnotationClassDiscoveryTest.php
index cbf2a16b374..b5729199297 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/CustomAnnotationClassDiscoveryTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/CustomAnnotationClassDiscoveryTest.php
@@ -46,8 +46,8 @@ class CustomAnnotationClassDiscoveryTest extends DiscoveryTestBase {
       'Drupal\plugin_test\Plugin\Annotation' => DRUPAL_ROOT . '/core/modules/system/tests/modules/plugin_test/lib',
     );
 
-    $this->discovery = new AnnotatedClassDiscovery('plugin_test/custom_annotation', $root_namespaces, $annotation_namespaces, 'Drupal\plugin_test\Plugin\Annotation\PluginExample');
-    $this->emptyDiscovery = new AnnotatedClassDiscovery('non_existing_module/non_existing_plugin_type', $root_namespaces, $annotation_namespaces, 'Drupal\plugin_test\Plugin\Annotation\PluginExample');
+    $this->discovery = new AnnotatedClassDiscovery('Plugin/plugin_test/custom_annotation', $root_namespaces, $annotation_namespaces, 'Drupal\plugin_test\Plugin\Annotation\PluginExample');
+    $this->emptyDiscovery = new AnnotatedClassDiscovery('Plugin/non_existing_module/non_existing_plugin_type', $root_namespaces, $annotation_namespaces, 'Drupal\plugin_test\Plugin\Annotation\PluginExample');
   }
 
 }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/CustomDirectoryAnnotatedClassDiscoveryTest.php b/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/CustomDirectoryAnnotatedClassDiscoveryTest.php
new file mode 100644
index 00000000000..c005300f847
--- /dev/null
+++ b/core/modules/system/lib/Drupal/system/Tests/Plugin/Discovery/CustomDirectoryAnnotatedClassDiscoveryTest.php
@@ -0,0 +1,48 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\system\Tests\Plugin\Discovery\CustomDirectoryAnnotatedClassDiscoveryTest.
+ */
+
+namespace Drupal\system\Tests\Plugin\Discovery;
+
+use Drupal\Core\Plugin\Discovery\AnnotatedClassDiscovery;
+
+/**
+ * Tests that plugins with annotated classes in a custom directory are correctly discovered.
+ */
+class CustomDirectoryAnnotatedClassDiscoveryTest extends DiscoveryTestBase {
+
+  public static function getInfo() {
+    return array(
+      'name' => 'Custom directory annotation class discovery',
+      'description' => 'Tests that plugins in a custom directory are correctly discovered using annotated classes.',
+      'group' => 'Plugin API',
+    );
+  }
+
+  protected function setUp() {
+    parent::setUp();
+
+    $this->expectedDefinitions = array(
+      'custom_example_1' => array(
+        'id' => 'custom_example_1',
+        'custom' => 'Tim',
+        'class' => 'Drupal\plugin_test\CustomDirectoryExample1',
+        'provider' => 'plugin_test',
+      ),
+      'custom_example_2' => array(
+        'id' => 'custom_example_2',
+        'custom' => 'Meghan',
+        'class' => 'Drupal\plugin_test\CustomDirectoryExample2',
+        'provider' => 'plugin_test',
+      ),
+    );
+    $namespaces = new \ArrayObject(array('Drupal\plugin_test' => DRUPAL_ROOT . '/core/modules/system/tests/modules/plugin_test/lib'));
+    $this->discovery = new AnnotatedClassDiscovery('', $namespaces);
+    $empty_namespaces = new \ArrayObject();
+    $this->emptyDiscovery = new AnnotatedClassDiscovery('', $empty_namespaces);
+  }
+
+}
diff --git a/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/CustomDirectoryExample1.php b/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/CustomDirectoryExample1.php
new file mode 100644
index 00000000000..b3be67cd1e6
--- /dev/null
+++ b/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/CustomDirectoryExample1.php
@@ -0,0 +1,20 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\plugin_test\CustomDirectoryExample1.
+ */
+
+namespace Drupal\plugin_test;
+
+use Drupal\Component\Annotation\Plugin;
+
+/**
+ * Provides a test plugin within a custom directory.
+ *
+ * @Plugin(
+ *   id = "custom_example_1",
+ *   custom = "Tim"
+ * )
+ */
+class CustomDirectoryExample1 {}
diff --git a/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/CustomDirectoryExample2.php b/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/CustomDirectoryExample2.php
new file mode 100644
index 00000000000..f15fb3a628a
--- /dev/null
+++ b/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/CustomDirectoryExample2.php
@@ -0,0 +1,20 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\plugin_test\CustomDirectoryExample2.
+ */
+
+namespace Drupal\plugin_test;
+
+use Drupal\Component\Annotation\Plugin;
+
+/**
+ * Provides a test plugin within a custom directory.
+ *
+ * @Plugin(
+ *   id = "custom_example_2",
+ *   custom = "Meghan"
+ * )
+ */
+class CustomDirectoryExample2 {}
diff --git a/core/modules/tour/lib/Drupal/tour/TipPluginManager.php b/core/modules/tour/lib/Drupal/tour/TipPluginManager.php
index f8b9f767e90..fb18d3b6ba6 100644
--- a/core/modules/tour/lib/Drupal/tour/TipPluginManager.php
+++ b/core/modules/tour/lib/Drupal/tour/TipPluginManager.php
@@ -32,7 +32,7 @@ class TipPluginManager extends DefaultPluginManager {
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, LanguageManager $language_manager, ModuleHandlerInterface $module_handler) {
     $annotation_namespaces = array('Drupal\tour\Annotation' => $namespaces['Drupal\tour']);
-    parent::__construct('tour/tip', $namespaces, $annotation_namespaces, 'Drupal\tour\Annotation\Tip');
+    parent::__construct('Plugin/tour/tip', $namespaces, $annotation_namespaces, 'Drupal\tour\Annotation\Tip');
 
     $this->alterInfo($module_handler, 'tour_tips_info');
     $this->setCacheBackend($cache_backend, $language_manager, 'tour');
diff --git a/core/modules/views/lib/Drupal/views/Plugin/ViewsPluginManager.php b/core/modules/views/lib/Drupal/views/Plugin/ViewsPluginManager.php
index 2b8c6a65c81..3e6099e96e0 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/ViewsPluginManager.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/ViewsPluginManager.php
@@ -31,7 +31,7 @@ class ViewsPluginManager extends PluginManagerBase {
    *   keyed by the corresponding namespace to look for plugin implementations,
    */
   public function __construct($type, \Traversable $namespaces) {
-    $this->discovery = new AnnotatedClassDiscovery("views/$type", $namespaces);
+    $this->discovery = new AnnotatedClassDiscovery("Plugin/views/$type", $namespaces);
     $this->discovery = new DerivativeDiscoveryDecorator($this->discovery);
     $this->discovery = new ProcessDecorator($this->discovery, array($this, 'processDefinition'));
     $this->discovery = new AlterDecorator($this->discovery, 'views_plugins_' . $type);