anchor.diff:    Stream asyncMap(convert(T event)) {
       StreamController controller;
       StreamSubscription subscription;
  -    controller = new StreamController(sync: true,
  +    controller = new StreamController(
         onListen: () {
           var add = controller.add;
           var addError = controller.addError;
  
positive.code:       },
        onPause: () { subscription.pause(); },
        onResume: () { subscription.resume(); },
        onCancel: () { subscription.cancel(); }
      );
      return controller.stream;
    }
  
negative.code:   Stream asyncExpand(Stream convert(T event)) {
      StreamController controller;
      StreamSubscription subscription;
      controller = new StreamController(
        onListen: () {
          subscription = this.listen(
              (T event) {
  