Added Area-Library, Triaged labels.

Stream.asBroadcastStream() is basically implemented as
new StreamController.broadcast(onListen: subscribeToPredecessor, onCancel: cancel);

We will add named onList and onCancel arguments on the asBroadcastStream method that could (partially) override the default behavior. By replacing the onCancel method one could then avoid the cancel and keep the stream open.

Removed Type-Defect label.
Added Type-Enhancement, Ready-to-implement, Accepted labels.

More details:
asBroadCast({ void onListen(StreamSubscription subscription), void onCancel(StreamSubscription subscription) });

The subscription will be a wrapped version of the actual subscription. It will not allow to change onData, onError and onDone.

Will the default onCancel cancel the underlying subscription? I think this is a bad idea. asBroadcastStream() should not have surprise behavior where it "dies" if it ever temporarily loses all of its listeners. If someone wants that behavior, they should specify it in their onCancel explicitly.

What about: asBroadcastStream({persistWithoutListeners: true});

or some well-named parameter?

The default onCancel will no longer cancel the original subscription.
We did this because there was no way for the user to signal that it didn't need the stream any more, so we picked the one signal that was there.

With the onCancel callback available, we can let the user decide when to cancel, and we don't need a hacky heuristic any more.

That does mean that some existing uses of asBroadcastStream will now not close the source stream, but wait for it to run dry. It's likely that it wasn't a deliberate choice before, so we don't think it will be a problem. Many uses of asBroadcastStream were on a StreamController's strem, and they are better rewritten as using a broadcast StreamController directly.

Added Started label.

Set owner to @lrhn.

Added Fixed label.