code: function getTabIndexValue(element) {
      if (!hasValidTabIndex(element)) {
          return null;
      }
      // See browser issue in Gecko https://bugzilla.mozilla.org/show_bug.cgi?id=1128054
      var /** @type {?} */ tabIndex = parseInt(element.getAttribute('tabindex'), 10);
      return isNaN(tabIndex) ? -1 : tabIndex;
  }
docs: Returns the parsed tabindex from the element attributes instead of returning the
  evaluated tabindex from the browsers defaults.
  @param {?} element
  @return {?}