Prevent submenu collapse in polymer's core-submenu

ik-submenu.html

<link rel="import" href="../../bower_components/polymer/polymer.html">
<link rel="import" href="../../bower_components/core-menu/core-submenu.html">

<!--
Similar to core-submenu, can be set to stay always open.
When selected, also selects it's first child.

    <ik-submenu icon="settings" label="Topics" alwaysOpened>
      <core-item label="Topic1"></core-item>
      <core-item label="Topic2"></core-item>
    </ik-submenu>
    
@element ik-submenu
@extends core-submenu
-->
<polymer-element name="ik-submenu" extends="core-submenu" attributes="alwaysOpened">
  <template>
    <shadow></shadow>
  </template>
  <script>
    (function () {
      'use strict';

      Polymer({
        /**
         * If true, the submenu will stay allways open; otherwise can collapse.
         *
         * @attribute alwaysOpened
         * @type boolean
         * @default false
         */
        alwaysOpened: false,
        get opened() {
          return this.alwaysOpened || this._opened;
        },
        set opened(v) {
          this._opened = v;
        },
        activate: function () {
          this.items[0].children[0].click();
        }
      });

    })();
  </script>
</polymer-element>