anchor.diff:  
     SimpleStream({bool isSync = false, bool runInZone = false})
         : _isSync = isSync,
  -        _runInZone = runInZone,
  -        super(const Stream<Null>.empty());
  +        _runInZone = runInZone;
   
     SimpleStream.broadcast(
         {bool isSync = false,
  
positive.code:   /// Remove a subscription.
    /// It marks a subscription for removal during a microtask to ensure that
    /// a subscription wasn't cancelled as a result of processing a callback.
    void _scheduleCleanup(SimpleStreamSubscription<T> subscription) {
      if (!_subscriptionRemoved) {
        _subscriptionRemoved = true;
        _cleanupStreams.add(this);
  
negative.code:       var stream = _stream;
        _doneCallback = null;
        _closeSubscription();
        stream._scheduleCleanup();
      }
      return null;
    }
  