anchor.diff:    final Flow flow = Flow.begin();
     Timeline.startSync('$debugLabel: start', flow: flow);
     final ReceivePort resultPort = ReceivePort();
  +  final ReceivePort exitPort = ReceivePort();
     final ReceivePort errorPort = ReceivePort();
     Timeline.finishSync();
     final Isolate isolate = await Isolate.spawn<_IsolateConfiguration<Q, FutureOr<R>>>(
  
positive.code:       flow.id,
      ),
      errorsAreFatal: true,
      onExit: resultPort.sendPort,
      onError: errorPort.sendPort,
    );
    final Completer<R> result = Completer<R>();
  
negative.code: }
  
  Future<void> _spawn<Q, R>(_IsolateConfiguration<Q, FutureOr<R>> configuration) async {
    final R result = await Timeline.timeSync(
      configuration.debugLabel,
      () async {
        final FutureOr<R> applicationResult = await configuration.apply();
        return await applicationResult;
      },
      flow: Flow.step(configuration.flowId),
    );
  