diff --git a/core/lib/Drupal/Core/Plugin/Discovery/HookDiscovery.php b/core/lib/Drupal/Core/Plugin/Discovery/HookDiscovery.php
index dce1741..035d402 100644
--- a/core/lib/Drupal/Core/Plugin/Discovery/HookDiscovery.php
+++ b/core/lib/Drupal/Core/Plugin/Discovery/HookDiscovery.php
@@ -8,6 +8,7 @@
 namespace Drupal\Core\Plugin\Discovery;
 
 use Drupal\Component\Plugin\Discovery\DiscoveryInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
 
 /**
  * Provides a hook-based plugin discovery class.
@@ -22,13 +23,23 @@ class HookDiscovery implements DiscoveryInterface {
   protected $hook;
 
   /**
+   * The module handler used to find and execute the plugin hook.
+   *
+   * @var \Drupal\Core\Extension\ModuleHandlerInterface
+   */
+  protected $moduleHandler;
+
+  /**
    * Constructs a Drupal\Core\Plugin\Discovery\HookDiscovery object.
    *
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   *   The module handler.
    * @param string $hook
    *   The Drupal hook that a module can implement in order to interface to
    *   this discovery class.
    */
-  function __construct($hook) {
+  function __construct(ModuleHandlerInterface $module_handler, $hook) {
+    $this->moduleHandler = $module_handler;
     $this->hook = $hook;
   }
 
@@ -45,9 +56,9 @@ public function getDefinition($plugin_id) {
    */
   public function getDefinitions() {
     $definitions = array();
-    foreach (\Drupal::moduleHandler()->getImplementations($this->hook) as $module) {
-      $function = $module . '_' . $this->hook;
-      foreach ($function() as $plugin_id => $definition) {
+    foreach ($this->moduleHandler->getImplementations($this->hook) as $module) {
+      $result = $this->moduleHandler->invoke($module, $this->hook);
+      foreach ($result as $plugin_id => $definition) {
         $definition['module'] = $module;
         $definitions[$plugin_id] = $definition;
       }
diff --git a/core/tests/Drupal/Tests/Core/Plugin/Discovery/HookDiscoveryTest.php b/core/tests/Drupal/Tests/Core/Plugin/Discovery/HookDiscoveryTest.php
new file mode 100644
index 0000000..3a120c8
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Plugin/Discovery/HookDiscoveryTest.php
@@ -0,0 +1,150 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\Core\Plugin\Discovery\HookDiscoveryTest.
+ */
+
+namespace Drupal\Tests\Core\Plugin\Discovery {
+
+use Drupal\Core\Plugin\Discovery\HookDiscovery;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * Tests the hook plugin discovery class.
+ *
+ * @group Drupal
+ * @group Plugin
+ *
+ * @see \Drupal\Core\Plugin\Discovery\HookDiscovery
+ */
+class HookDiscoveryTest extends UnitTestCase {
+
+  /**
+   * The mocked module handler.
+   *
+   * @var \Drupal\Core\Extension\ModuleHandlerInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $moduleHandler;
+
+  /**
+   * The tested hook discovery.
+   *
+   * @var \Drupal\Core\Plugin\Discovery\HookDiscovery
+   */
+  protected $hookDiscovery;
+
+  public static function getInfo() {
+    return array(
+      'name' => 'Hook Discovery',
+      'description' => 'Tests the hook plugin discovery class.',
+      'group' => 'Plugin',
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    $this->moduleHandler = $this->getMock('Drupal\Core\Extension\ModuleHandlerInterface');
+    $this->hookDiscovery = new HookDiscovery($this->moduleHandler, 'test_plugin');
+  }
+
+  /**
+   * Tests the getDefinitions() method without any plugins.
+   *
+   * @see \Drupal\Core\Plugin\Discovery::getDefinitions()
+   */
+  public function testGetDefinitionsWithoutPlugins() {
+    $this->moduleHandler->expects($this->once())
+      ->method('getImplementations')
+      ->with('test_plugin')
+      ->will($this->returnValue(array()));
+
+    $this->assertCount(0, $this->hookDiscovery->getDefinitions());
+  }
+
+  /**
+   * Tests the getDefinitions() method with some plugins.
+   *
+   * @see \Drupal\Core\Plugin\Discovery::getDefinitions()
+   */
+  public function testGetDefinitions() {
+    $this->moduleHandler->expects($this->once())
+      ->method('getImplementations')
+      ->with('test_plugin')
+      ->will($this->returnValue(array('hook_discovery_test', 'hook_discovery_test2')));
+
+    $this->moduleHandler->expects($this->at(1))
+      ->method('invoke')
+      ->with('hook_discovery_test', 'test_plugin')
+      ->will($this->returnValue(hook_discovery_test_test_plugin()));
+    $this->moduleHandler->expects($this->at(2))
+      ->method('invoke')
+      ->with('hook_discovery_test2', 'test_plugin')
+      ->will($this->returnValue(hook_discovery_test2_test_plugin()));
+
+    $definitions = $this->hookDiscovery->getDefinitions();
+
+    $this->assertCount(3, $definitions);
+    $this->assertEquals($definitions['test_id_1']['class'], 'Drupal\plugin_test\Plugin\plugin_test\fruit\Apple');
+    $this->assertEquals($definitions['test_id_2']['class'], 'Drupal\plugin_test\Plugin\plugin_test\fruit\Orange');
+    $this->assertEquals($definitions['test_id_3']['class'], 'Drupal\plugin_test\Plugin\plugin_test\fruit\Cherry');
+
+    // Ensure that the module was set.
+    $this->assertEquals($definitions['test_id_1']['module'], 'hook_discovery_test');
+    $this->assertEquals($definitions['test_id_2']['module'], 'hook_discovery_test');
+    $this->assertEquals($definitions['test_id_3']['module'], 'hook_discovery_test2');
+  }
+
+  /**
+   * Tests the getDefinition method with some plugins.
+   *
+   * @see \Drupal\Core\Plugin\Discovery::getDefinition()
+   */
+  public function testGetDefinition() {
+    $this->moduleHandler->expects($this->exactly(4))
+      ->method('getImplementations')
+      ->with('test_plugin')
+      ->will($this->returnValue(array('hook_discovery_test', 'hook_discovery_test2')));
+
+    $this->moduleHandler->expects($this->any())
+      ->method('invoke')
+      ->will($this->returnValueMap(array(
+          array('hook_discovery_test', 'test_plugin', array(), hook_discovery_test_test_plugin()),
+          array('hook_discovery_test2', 'test_plugin', array(), hook_discovery_test2_test_plugin()),
+        )
+      ));
+
+    $this->assertNull($this->hookDiscovery->getDefinition('test_non_existant'));
+
+    $plugin_definition = $this->hookDiscovery->getDefinition('test_id_1');
+    $this->assertEquals($plugin_definition['class'], 'Drupal\plugin_test\Plugin\plugin_test\fruit\Apple');
+    $this->assertEquals($plugin_definition['module'], 'hook_discovery_test');
+
+    $plugin_definition = $this->hookDiscovery->getDefinition('test_id_2');
+    $this->assertEquals($plugin_definition['class'], 'Drupal\plugin_test\Plugin\plugin_test\fruit\Orange');
+    $this->assertEquals($plugin_definition['module'], 'hook_discovery_test');
+
+    $plugin_definition = $this->hookDiscovery->getDefinition('test_id_3');
+    $this->assertEquals($plugin_definition['class'], 'Drupal\plugin_test\Plugin\plugin_test\fruit\Cherry');
+    $this->assertEquals($plugin_definition['module'], 'hook_discovery_test2');
+  }
+
+}
+
+}
+
+namespace {
+  function hook_discovery_test_test_plugin() {
+    return array(
+      'test_id_1' => array('class' => 'Drupal\plugin_test\Plugin\plugin_test\fruit\Apple'),
+      'test_id_2' => array('class' => 'Drupal\plugin_test\Plugin\plugin_test\fruit\Orange'),
+    );
+  }
+  function hook_discovery_test2_test_plugin() {
+    return array(
+      'test_id_3' => array('class' => 'Drupal\plugin_test\Plugin\plugin_test\fruit\Cherry'),
+    );
+  }
+}