boolean}
       * @private
       */
      _collectTabbableNodes: function(node, result) {
        // If not an element or not visible, no need to explore children.
        if (node.nodeType !== Node.ELEMENT_NODE || !this._isVisible(node)) {
          return false;
        }
        var element = /** @type {!HTMLElement} */ (node);
        var tabIndex = this._normalizedTabIndex(element);
        var needsSort = tabIndex > 0;
        if (tabIndex >= 0) {
          result.push(element);
        }

        //