code: function(event, key, prevChild) {
            var eventData = {event: event, key: key};
            if( angular.isDefined(prevChild) ) {
              eventData.prevChild = prevChild;
            }
            angular.forEach(this._observers, function(parts) {
              parts[0].call(parts[1], eventData);
            });
          }
docs: Used to trigger notifications for listeners registered using $watch. This method is
  typically invoked internally by the $$process method.
  
  @param {string} event
  @param {string} key
  @param {string} [prevChild]
  @protected