diff --git a/core/lib/Drupal/Component/Plugin/PluginBag.php b/core/lib/Drupal/Component/Plugin/PluginBag.php
index 671a70a..9487d8c 100644
--- a/core/lib/Drupal/Component/Plugin/PluginBag.php
+++ b/core/lib/Drupal/Component/Plugin/PluginBag.php
@@ -9,6 +9,9 @@
 
 /**
  * Defines an object which stores multiple plugin instances to lazy load them.
+ *
+ * The \ArrayAccess implementation is only for backwards compatibility, it is
+ * deprecated and should not be used by new code.
  */
 abstract class PluginBag implements \ArrayAccess, \Iterator, \Countable {
 
@@ -42,7 +45,56 @@ public function clear() {
   }
 
   /**
+   * Determines if a plugin instance exists.
+   *
+   * @param string $instance_id
+   *   The ID of the plugin instance to check.
+   *
+   * @return bool
+   *   TRUE if the plugin instance exists, FALSE otherwise.
+   */
+  public function has($instance_id) {
+    return isset($this->pluginInstances[$instance_id]) || isset($this->instanceIDs[$instance_id]);
+  }
+
+  /**
+   * Retrieves a plugin instance, initializing it if necessary.
+   *
+   * @param string $instance_id
+   *   The ID of the plugin instance being retrieved.
+   */
+  public function get($instance_id) {
+    if (!isset($this->pluginInstances[$instance_id])) {
+      $this->initializePlugin($instance_id);
+    }
+    return $this->pluginInstances[$instance_id];
+  }
+
+  /**
+   * Stores an initialized plugin.
+   *
+   * @param string $instance_id
+   *   The ID of the plugin instance being stored.
+   * @param mixed $value
+   *   An instantiated plugin.
+   */
+  public function set($instance_id, $value) {
+    $this->pluginInstances[$instance_id] = $value;
+  }
+
+  /**
+   * Removes an initialized plugin.
+   *
+   * The plugin can still be used, it will be reinitialized.
+   */
+  public function remove($instance_id) {
+    unset($this->pluginInstances[$instance_id]);
+  }
+
+  /**
    * Implements \ArrayAccess::offsetExists().
+   *
+   * This is deprecated, use \Drupal\Component\Plugin\PluginBag::has().
    */
   public function offsetExists($offset) {
     return isset($this->pluginInstances[$offset]) || isset($this->instanceIDs[$offset]);
@@ -50,6 +102,8 @@ public function offsetExists($offset) {
 
   /**
    * Implements \ArrayAccess::offsetGet().
+   *
+   * This is deprecated, use \Drupal\Component\Plugin\PluginBag::get().
    */
   public function offsetGet($offset) {
     if (!isset($this->pluginInstances[$offset])) {
@@ -60,6 +114,8 @@ public function offsetGet($offset) {
 
   /**
    * Implements \ArrayAccess::offsetSet().
+   *
+   * This is deprecated, use \Drupal\Component\Plugin\PluginBag::set().
    */
   public function offsetSet($offset, $value) {
     $this->pluginInstances[$offset] = $value;
@@ -67,6 +123,8 @@ public function offsetSet($offset, $value) {
 
   /**
    * Implements \ArrayAccess::offsetUnset().
+   *
+   * This is deprecated, use \Drupal\Component\Plugin\PluginBag::remove().
    */
   public function offsetUnset($offset) {
     unset($this->pluginInstances[$offset]);