be focused even if they have [disabled] attribute.
        return matches.call(element,
          'a[href], area[href], iframe, [tabindex], [contentEditable]');
      },

      /**
       * Returns if a element is tabbable. To be tabbable, a element must be
       * focusable, visible, and with a tabindex !== -1.
       * @param {!HTMLElement} element
       * @return {boolean}
       */
      isTabbable: function(element) {
        return this.isFocusable(element) &&
          matches.call(element, ':not([