function linkFn(scope, element, attrs) {
      if (attrs.modalInClass) {
        $animate.addClass(element, attrs.modalInClass);

        scope.$on($modalStack.NOW_CLOSING_EVENT, function(e, setIsAsync) {
          var done = setIsAsync();
          if (scope.modalOptions.animation) {
            $animate.removeClass(element, attrs.modalInClass).then(done);
          } else {
            done();
          }
        });
      }
    }
A helper directive for the $modal service. It creates a backdrop element.