diff -u b/core/lib/Drupal/Component/Plugin/PluginBag.php b/core/lib/Drupal/Component/Plugin/PluginBag.php
--- b/core/lib/Drupal/Component/Plugin/PluginBag.php
+++ b/core/lib/Drupal/Component/Plugin/PluginBag.php
@@ -8,7 +8,7 @@
 namespace Drupal\Component\Plugin;
 
 /**
- * Defines an object which stores multiple plugin instances to allow lazy initialization.
+ * Defines an object which stores multiple plugin instances to lazy load them.
  */
 abstract class PluginBag implements \ArrayAccess, \Iterator, \Countable {
 
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/system/lib/Drupal/system/Tests/Plugin/PluginBagTest.php
@@ -0,0 +1,63 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\system\Tests\Plugin\PluginBagTest.
+ */
+
+namespace Drupal\system\Tests\Plugin;
+
+use Drupal\plugin_test\Plugin\TestPluginBag;
+
+/**
+ * Tests the generic plugin bag.
+ *
+ * @see \Drupal\Component\Plugin\PluginBag
+ * @see \Drupal\plugin_test\Plugin\TestPluginBag
+ */
+class PluginBagTest extends PluginTestBase {
+
+  public static function getInfo() {
+    return array(
+      'name' => 'Plugin Bag',
+      'description' => 'Tests the generic plugin bag.',
+      'group' => 'Plugin API',
+    );
+  }
+
+  /**
+   * Tests the plugin bag.
+   */
+  protected function testPluginBag() {
+    // Setup the plugin bag as well as the available plugin definitions.
+    $plugin_bag = new TestPluginBag($this->mockBlockManager);
+    $definitions = $this->mockBlockManager->getDefinitions();
+    $first_instance_id = key($definitions);
+
+    foreach ($definitions as $instance_id => $definition) {
+      $this->assertTrue(isset($plugin_bag[$instance_id]), format_string('Plugin instance @instance_id exits on the bag', array('@instance_id' => $instance_id)));
+    }
+
+    // A non existing instance_id shouldn't exists on the bag.
+    $random_name = $this->randomName();
+    $this->assertFalse(isset($plugin_bag[$random_name]), 'A random instance_id should not exist on the plugin bag.');
+
+    // Set a new plugin instance to the bag, to test offsetSet.
+    $plugin_bag[$random_name] = $this->mockBlockManager->createInstance($first_instance_id, array());
+    $this->assertTrue(isset($plugin_bag[$random_name]), 'A random instance_id should exist after manual setting on the plugin bag.');
+
+    // Remove the previous added element and check whether it still exists.
+    unset($plugin_bag[$random_name]);
+    $this->assertFalse(isset($plugin_bag[$random_name]), 'A random instance_id should not exist on the plugin bag after removing.');
+
+    // Check that iterating over the plugins work.
+    $expected_instance_ids = array_keys($definitions);
+    $counter = 0;
+    foreach ($plugin_bag as $instance_id => $plugin) {
+      $this->assertEqual($expected_instance_ids[$counter], $instance_id, format_string('The iteration works as expected for plugin instance @instance_id', array('@instance_id' => $instance_id)));
+      $counter++;
+    }
+
+    $this->assertEqual(count($plugin_bag), count($expected_instance_ids), 'The amount of items in plugin bag is as expected.');
+  }
+}
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/system/tests/modules/plugin_test/lib/Drupal/plugin_test/Plugin/TestPluginBag.php
@@ -0,0 +1,49 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\plugin_test\Plugin\TestPluginBag.
+ */
+
+namespace Drupal\plugin_test\Plugin;
+
+use Drupal\Component\Plugin\PluginBag;
+use Drupal\Component\Plugin\PluginManagerInterface;
+
+/**
+ * Defines a plugin bag which uses fruit plugins.
+ */
+class TestPluginBag extends PluginBag {
+
+  /**
+   * Stores the plugin manager used by this bag.
+   *
+   * @var \Drupal\Component\Plugin\PluginManagerInterface
+   */
+  protected $manager;
+
+  /**
+   * Constructs a TestPluginBag object.
+   *
+   * @param \Drupal\Component\Plugin\PluginManagerInterface $manager
+   *   The plugin manager that handles test plugins.
+   */
+  public function __construct(PluginManagerInterface $manager) {
+    $this->manager = $manager;
+
+    $this->instanceIDs = drupal_map_assoc(array_keys($this->manager->getDefinitions()));
+  }
+
+  /**
+   * Implements \Drupal\Component\Plugin\PluginBag::initializePlugin().
+   */
+  protected function initializePlugin($instance_id) {
+    // If the plugin was initialized before, just return.
+    if (isset($this->pluginInstances[$instance_id])) {
+      return;
+    }
+
+    $this->pluginInstances[$instance_id] = $this->manager->createInstance($instance_id, array());
+  }
+
+}