anchor.diff:    /**
      * Connects a callback to this signal.
      */
  -  on(Callback callback) => subscriptions.add(callback);
  +  on(dynamic callback(T argument)) => subscriptions.add(callback);
   
   }
positive.code:  *     > Results of callbacks: [1, 1]
   *
   */
  class Signal {
  
    final List<Callback> subscriptions = <Callback>[];
  
  
negative.code:    * The return value is a future which will fire with a list of results from
     * all signal handlers.
     */
    Future<List> emit(T argument) {
      var results = <Future>[];
      for (Callback subscription in subscriptions) {
        var result = subscription(argument);
  