anchor.diff:        }
         asyncCount++;
         parent.scheduleMicrotask(zone, () {
  -        trace.record(new UserCallbackAsyncEnterEvent());
  +        trace.record(new UserCallbackAsyncEnterEvent(
  +            response: response, error: error));
           try {
             f();
           } finally {
  -          trace.record(new UserCallbackAsyncExitEvent());
  +          trace.record(new UserCallbackAsyncExitEvent(
  +              response: response, error: error));
             asyncCount--;
             if (asyncCount == 0) {
               inOnDone = true;
  
positive.code:   UserCallbackTracingTransformer() : super();
  
    void handleData(Response response, EventSink<Response> sink, Trace trace) {
      trace.record(new UserCallbackQueuedEvent());
      _openCallbacks++;
      _runZonedWithOnDone(() => sink.add(response), () {
        trace.record(new UserCallbackDoneEvent());
        _openCallbacks--;
        if (_openCallbacks == 0 && _closed && !_sentDone) {
          trace.record(new RequestOverEvent());
          _sentDone = true;
        }
      }, trace);
    }
  
    void handleError(error, EventSink<Response> sink, Trace trace) {
      trace.record(new UserCallbackQueuedEvent());
      _openCallbacks++;
      _runZonedWithOnDone(() {
        sink.addError(error);
      }, () {
        trace.record(new UserCallbackDoneEvent());
        _openCallbacks--;
        if (_openCallbacks == 0 && _closed && !_sentDone) {
          trace.record(new RequestOverEvent());
          _sentDone = true;
        }
      }, trace);
    }
  
    void handleDone(EventSink<Response> sink, Trace trace) {
  
negative.code: // this can be optimized to return a const-constructed [StreamTransformer].
  typedef StreamTransformer<Response, Response> StreamTransformerFactory(Request request, Trace trace);
  
  _runZonedWithOnDone(fn, onDone, trace, {response: null, error: null}) {
    // Initial count is 1 due to running 'fn'. This makes it work out
    // nicely if 'fn' itself throws an Exception.
    var asyncCount = 1;
  