anchor.diff:    /// Using array as the assumption is that there will mainly be limited number
     /// of subscribers and overhead of a map is more.  It can always be made
     /// smarter.  If it is null, it means that the stream is closed.
  -  List<SimpleStreamSubscription<T>> _subscriptions = [];
  +  List<SimpleStreamSubscription<T>> _subscriptions = const [];
   
     /// List of items to send to avoid scheduling multiple micro tasks for each
     /// item to be sent.
  
positive.code:   }
  
    @override
    StreamSubscription<T> listen(onData, {onError, onDone, cancelOnError}) {
      SimpleStreamSubscription<T> sub = super.listen(onData,
          onError: onError, onDone: onDone, cancelOnError: cancelOnError);
      if (_lastItem != null) {
  
negative.code:   }
  
    @override
    StreamSubscription<T> listen(void onData(T event),
        {Function onError, void onDone(), bool cancelOnError}) {
      // Don't allow listening to a closed stream, it will throw exception in
      // non checked mode since subscriptions will be null once the stream is
      // closed.
  