DoStreamTransformer not behaving as expected

[rbellens]

The DoStreamTransformer seems to be the only transformer that uses forwardStream with listenOnlyOnce argument equal to true. Don't exactly now what is the purpose of this argument, but it leads to some unexpected behavior in the following case:

var s = BehaviorSubject<dynamic>()..add(1);

var stream = s.doOnData((_) {});

var s1 = stream.listen((v) {
  print('$v'); // prints 1
});

await stream.first; // completes with value 1

await stream.first; // NEVER COMPLETES

await s1.cancel();

[hoc081098]

I have seen this issue too, but if we use Stream.multi for doOnXXX, doOnXXX's callback will fire many times even source Stream is broadcast. https://github.com/ReactiveX/rxdart/pull/412#issue-577289261