mputed style.
        var style = element.style;
        if (style.visibility !== 'hidden' && style.display !== 'none') {
          style = window.getComputedStyle(element);
          return (style.visibility !== 'hidden' && style.display !== 'none');
        }
        return false;
      },

      /**
       * Sorts an array of tabbable elements by tabindex. Returns a new array.
       * @param {!Array<!HTMLElement>} tabbables
       * @return {!Array<!HTMLElement>}
       * @private
       */
      _sortB