diff --git a/core/lib/Drupal/Component/Plugin/Context/Context.php b/core/lib/Drupal/Component/Plugin/Context/Context.php
index b377c75..ce8ccf4 100644
--- a/core/lib/Drupal/Component/Plugin/Context/Context.php
+++ b/core/lib/Drupal/Component/Plugin/Context/Context.php
@@ -22,7 +22,7 @@ class Context {
   protected $context;
 
   /**
-   * The PSR-0 class or an array definition to which context should conform.
+   * The class name or an array definition to which a context must conform.
    *
    * @var mixed
    *  A string or array.
diff --git a/core/modules/system/lib/Drupal/system/Tests/Plugin/ContextPluginTest.php b/core/modules/system/lib/Drupal/system/Tests/Plugin/ContextPluginTest.php
index b08aadb..19a53f9 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Plugin/ContextPluginTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Plugin/ContextPluginTest.php
@@ -7,16 +7,17 @@
 
 namespace Drupal\system\Tests\Plugin;
 
-use Drupal\simpletest\WebTestBase;
+use Drupal\simpletest\UnitTestBase;
 use Drupal\plugin_test\Plugin\MockBlockManager;
+use Drupal\plugin_test\MockNode;
+use Drupal\plugin_test\MockUser;
+use Drupal\Component\Plugin\Exception\PluginException;
 use Drupal\Component\Plugin\Exception\ContextException;
 
 /**
  * Tests that derivative plugins are correctly discovered.
  */
-class ContextPluginTest extends WebTestBase {
-
-  public static $modules = array('user', 'node');
+class ContextPluginTest extends UnitTestBase {
 
   public static function getInfo() {
     return array(
@@ -34,15 +35,60 @@ function testContext() {
     $manager = new MockBlockManager();
     $plugin = $manager->createInstance('user_name');
     // Create a node, add it as context, catch the exception.
-    $node = entity_create('node', array('title' => $name));
+    $node = new MockNode(array('title' => $name));
+
+    // Try to get a valid context that has not been set.
+    try {
+      $plugin->getContext('user');
+    }
+    catch (PluginException $e) {
+      $this->assertEqual($e->getMessage(), 'The user context is not yet set.');
+    }
+
+    // Try to call a method of the plugin that requires context before it has
+    // been set.
+    try {
+      $plugin->getTitle();
+    }
+    catch (PluginException $e) {
+      $this->assertEqual($e->getMessage(), 'The user context is not yet set.');
+    }
+
+    // Try to get a context that is not valid.
+    try {
+      $plugin->getContext('node');
+    }
+    catch (PluginException $e) {
+      $this->assertEqual($e->getMessage(), 'The node context is not a valid context.');
+    }
+
+    // Try to pass the wrong class type as a context.
     try {
       $plugin->setContext('user', $node);
     }
     catch (ContextException $e) {
-      $this->assertEqual($e->getMessage(), 'The context passed was not an instance of Drupal\user\Plugin\Core\Entity\User.');
+      $this->assertEqual($e->getMessage(), 'The context passed was not an instance of Drupal\plugin_test\MockUser.');
     }
-    $user = entity_create('user', array('name' => $name));
+
+    // Set an appropriate context appropriately and check to make sure its
+    // methods work as expected.
+    $user = new MockUser(array('name' => $name));
     $plugin->setContext('user', $user);
     $this->assertEqual($user->label(), $plugin->getTitle());
+
+    // Test the getContextDefinitions() method.
+    $this->assertIdentical($plugin->getContextDefinitions(), array('user' => 'Drupal\plugin_test\MockUser'));
+
+    // Test the getContextDefinition() method for a valid context.
+    $this->assertEqual($plugin->getContextDefinition('user'), 'Drupal\plugin_test\MockUser');
+
+    // Test the getContextDefinition() method for an invalid context.
+    try {
+      $plugin->getContextDefinition('node');
+    }
+    catch (PluginException $e) {
+      $this->assertEqual($e->getMessage(), 'The node context is not a valid context.');
+    }
+
   }
 }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Plugin/PluginTestBase.php b/core/modules/system/lib/Drupal/system/Tests/Plugin/PluginTestBase.php
index b62878e..f80daf8 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Plugin/PluginTestBase.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Plugin/PluginTestBase.php
@@ -74,7 +74,7 @@ public function setUp() {
         'label' => 'User Name',
         'class' => 'Drupal\plugin_test\Plugin\plugin_test\mock_block\MockUserNameBlock',
         'context' => array(
-          'user' => 'Drupal\user\Plugin\Core\Entity\User'
+          'user' => 'Drupal\plugin_test\MockUser'
         ),
       ),
     );
diff --git a/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/MockEntity.php b/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/MockEntity.php
new file mode 100644
index 0000000..941330d
--- /dev/null
+++ b/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/MockEntity.php
@@ -0,0 +1,26 @@
+<?php
+
+/**
+ * @file
+ * Definition of Drupal\plugin_test\MockEntity.
+ */
+
+namespace Drupal\plugin_test;
+
+abstract class MockEntity {
+
+  /**
+   * The title of the mock entity.
+   *
+   * @var string
+   *  A string.
+   */
+  protected $title;
+
+  /**
+   * A simple label function.
+   */
+  public function label() {
+    return $this->title;
+  }
+}
diff --git a/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/MockNode.php b/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/MockNode.php
new file mode 100644
index 0000000..fba6a65
--- /dev/null
+++ b/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/MockNode.php
@@ -0,0 +1,18 @@
+<?php
+
+/**
+ * @file
+ * Definition of Drupal\plugin_test\MockNode.
+ */
+
+namespace Drupal\plugin_test;
+
+class MockNode extends MockEntity {
+
+  /**
+   * A simple label function.
+   */
+  public function __construct(array $values = array()) {
+    $this->title = isset($values['title']) ? $values['title'] : '';
+  }
+}
diff --git a/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/MockUser.php b/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/MockUser.php
new file mode 100644
index 0000000..374ec95
--- /dev/null
+++ b/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/MockUser.php
@@ -0,0 +1,18 @@
+<?php
+
+/**
+ * @file
+ * Definition of Drupal\plugin_test\MockUser.
+ */
+
+namespace Drupal\plugin_test;
+
+class MockUser extends MockEntity {
+
+  /**
+   * A simple label function.
+   */
+  public function __construct(array $values = array()) {
+    $this->title = isset($values['name']) ? $values['name'] : '';
+  }
+}
diff --git a/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/Plugin/MockBlockManager.php b/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/Plugin/MockBlockManager.php
index 7f65b0c..2ba5dd9 100644
--- a/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/Plugin/MockBlockManager.php
+++ b/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/Plugin/MockBlockManager.php
@@ -72,7 +72,7 @@ public function __construct() {
       'label' => t('User Name'),
       'class' => 'Drupal\plugin_test\Plugin\plugin_test\mock_block\MockUserNameBlock',
       'context' => array(
-        'user' => 'Drupal\user\Plugin\Core\Entity\User'
+        'user' => 'Drupal\plugin_test\MockUser'
       ),
     ));