addRx method
Add an Rx to this composite
Implementation
@protected
void addRx(Rx rx) {
if (_subs[identityHashCode(rx)] != null) return;
// Pana was complaining about this for no reason
// ignore: cancel_subscriptions
final sub = rx.stream.listen((_) => notify());
_subs[identityHashCode(rx)] = sub;
}