diff --git a/core/modules/system/lib/Drupal/system/Tests/Plugin/PluginBagTest.php b/core/modules/system/lib/Drupal/system/Tests/Plugin/PluginBagTest.php
index 207beb8..892ca23 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Plugin/PluginBagTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Plugin/PluginBagTest.php
@@ -8,6 +8,7 @@
 namespace Drupal\system\Tests\Plugin;
 
 use Drupal\plugin_test\Plugin\TestPluginBag;
+use Drupal\plugin_test\Plugin\plugin_test\mock_block\MockTestPluginInterface;
 
 /**
  * Tests the generic plugin bag.
@@ -36,19 +37,32 @@ protected function testPluginBag() {
 
     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)));
+      $this->assertTrue($plugin_bag->has($instance_id), format_string('Plugin instance @instance_id exits on the bag', array('@instance_id' => $instance_id)));
+      $this->assertTrue($plugin_bag[$instance_id] instanceof $definition['class'], 'Getting the plugin from the bag worked.');
+      $this->assertTrue($plugin_bag->get($instance_id) instanceof $definition['class'], 'Getting the plugin from the bag worked.');
     }
 
     // A non existing instance_id shouldn't exist on the bag.
     $random_name = $this->randomName();
+    $random_name_2 = $this->randomName();
     $this->assertFalse(isset($plugin_bag[$random_name]), 'A random instance_id should not exist on the plugin bag.');
+    $this->assertFalse($plugin_bag->has($random_name_2), '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());
+    $plugin_bag->set($random_name_2, $this->mockBlockManager->createInstance($first_instance_id, array()));
     $this->assertTrue(isset($plugin_bag[$random_name]), 'A random instance_id should exist after manual setting on the plugin bag.');
+    $this->assertTrue(isset($plugin_bag[$random_name_2]), 'A random instance_id should exist after manual setting on the plugin bag.');
+    $this->assertTrue($plugin_bag->has($random_name), 'A random instance_id should exist after manual setting on the plugin bag.');
+    $this->assertTrue($plugin_bag->has($random_name_2), '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]);
+    $plugin_bag->remove($random_name_2);
     $this->assertFalse(isset($plugin_bag[$random_name]), 'A random instance_id should not exist on the plugin bag after removing.');
+    $this->assertFalse(isset($plugin_bag[$random_name_2]), 'A random instance_id should not exist on the plugin bag after removing.');
+    $this->assertFalse($plugin_bag->has($random_name), 'A random instance_id should not exist on the plugin bag after removing.');
+    $this->assertFalse($plugin_bag->has($random_name_2), 'A random instance_id should not exist on the plugin bag after removing.');
 
     // Test that iterating over the plugins work.
     $expected_instance_ids = array_keys($definitions);