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:     throw new UnsupportedError('Not supported.');
    }
  }
  
negative.code:     }
      var sub = new SimpleStreamSubscription<T>(
          this, onData, onDone, onError, cancelOnError, contextFunc);
      if (_subscriptions.isEmpty) {
        _subscriptions = [sub];
      } else {
        _subscriptions.add(sub);
      }
      if (_onListen != null && _subscriptions.length == 1) {
        _onListen(sub);
      }
  