Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/toolbar/js/models/MenuModel.js

@@ -0,0 +1,33 @@
+/**
+ * @file
+ * A Backbone Model for collapsible menus.
+ */
+
+(function (Backbone, Drupal) {
+
+  "use strict";
+
+  /**
+   * Backbone Model for collapsible menus.
+   *
+   * @constructor
+   *
+   * @augments Backbone.Model
+   */
+  Drupal.toolbar.MenuModel = Backbone.Model.extend(/** @lends Drupal.toolbar.MenuModel# */{
+
+    /**
+     * @type {object}
+     *
+     * @prop {object} subtrees
+     */
+    defaults: /** @lends Drupal.toolbar.MenuModel# */{
+
+      /**
+       * @type {object}
+       */
+      subtrees: {}
+    }
+  });
+
+}(Backbone, Drupal));