Switch to SingleSubscriptionTransformer<S> about async HOT 4 OPEN

/// A transformer that converts a broadcast stream into a single-subscription
/// stream.
///
/// This buffers the broadcast stream's events, which means that it starts
/// listening to a stream as soon as it's bound.
class SingleSubscriptionTransformer<S> extends StreamTransformerBase<S, S> {
  const SingleSubscriptionTransformer();

  @override
  Stream<S> bind(Stream<S> stream) {
    StreamSubscription<S> subscription;
    var controller =
        StreamController<S>(sync: true, onCancel: () => subscription.cancel());
    subscription = stream.listen(controller.add,
        onError: controller.addError, onDone: controller.close);
    return controller.stream;
  }
}

  @override
  Stream<S> bind(Stream<S> stream) {
    var controller = StreamController<S>(sync: true);
    controller.addStream(stream).whenComplete(controller.close);
    return controller.stream;
  }