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

core/modules/toolbar/js/views/MenuVisualView.js

@@ -0,0 +1,46 @@
+/**
+ * @file
+ * A Backbone view for the collapsible menus.
+ */
+
+(function ($, Backbone, Drupal) {
+
+  "use strict";
+
+  Drupal.toolbar.MenuVisualView = Backbone.View.extend(/** @lends Drupal.toolbar.MenuVisualView# */{
+
+    /**
+     * Backbone View for collapsible menus.
+     *
+     * @constructs
+     *
+     * @augments Backbone.View
+     */
+    initialize: function () {
+      this.listenTo(this.model, 'change:subtrees', this.render);
+    },
+
+    /**
+     * @inheritdoc
+     */
+    render: function () {
+      var subtrees = this.model.get('subtrees');
+      // Add subtrees.
+      for (var id in subtrees) {
+        if (subtrees.hasOwnProperty(id)) {
+          this.$el
+            .find('#toolbar-link-' + id)
+            .once('toolbar-subtrees')
+            .after(subtrees[id]);
+        }
+      }
+      // Render the main menu as a nested, collapsible accordion.
+      if ('drupalToolbarMenu' in $.fn) {
+        this.$el
+          .children('.toolbar-menu')
+          .drupalToolbarMenu();
+      }
+    }
+  });
+
+}(jQuery, Backbone, Drupal));