code: function(newClasses, oldClasses) {
          var toAdd = tokenDifference(newClasses, oldClasses);
          if (toAdd && toAdd.length) {
            $animate.addClass(this.$$element, toAdd);
          }
  
          var toRemove = tokenDifference(oldClasses, newClasses);
          if (toRemove && toRemove.length) {
            $animate.removeClass(this.$$element, toRemove);
          }
        }
docs: @ngdoc method
  @name $compile.directive.Attributes#$updateClass
  @kind function
  
  @description
  Adds and removes the appropriate CSS class values to the element based on the difference
  between the new and old CSS class values (specified as newClasses and oldClasses).
  
  @param {string} newClasses The current CSS className value
  @param {string} oldClasses The former CSS className value