/ Ensure method is always invoked to collect tabbable children.
          needsSort = this._collectTabbableNodes(children[i], result) || needsSort;
        }
        return needsSort;
      },

      /**
       * Returns false if the element has `visibility: hidden` or `display: none`
       * @param {!HTMLElement} element
       * @return {boolean}
       * @private
       */
      _isVisible: function(element) {
        // Check inline style first to save a re-flow. If looks good, check also
        // co