     * @type {Array<Node>}
     * @protected
     */
    get _focusableNodes() {
      // Elements that can be focused even if they have [disabled] attribute.
      var FOCUSABLE_WITH_DISABLED = [
        'a[href]',
        'area[href]',
        'iframe',
        '[tabindex]',
        '[contentEditable=true]'
      ];

      // Elements that cannot be focused if they have [disabled] attribute.
      var FOCUSABLE_WITHOUT_DISABLED = [
        'input',
        'select',
        'textarea',
        'button'
  