anchor.diff:      });
     }
   
  -  ServiceImpl getService(String serviceId) {
  -    return new ExampleServiceImpl(this);
  +  void _registerServiceImpl(ServiceImpl serviceImplementation) {
  +    _serviceImplsById[serviceImplementation.serviceId] =
  +        serviceImplementation;
  +  }
  +
  +  ServiceImpl getServiceImpl(String serviceId) {
  +    return _serviceImplsById[serviceId];
     }
   
     Future<ServiceActionEvent> _handleMessage(ServiceActionEvent event) {
  
positive.code:   Stream<ServiceActionEvent> onResponseMessage;
  
    ChromeService chromeService;
  
    Future<ServiceActionEvent> _onResponseByCallId(String callId) {
      Completer<ServiceActionEvent> completer =
  
negative.code:     Completer<ServiceActionEvent> completer =
          new Completer<ServiceActionEvent>();
  
      ServiceImpl service = getServiceImpl(event.serviceId);
      /*%TRACE3*/ print("""(4> 2/13/14): event.serviceId: ${event.serviceId}"""); // TRACE%
      /*%TRACE3*/ print("""(4> 2/13/14): service: ${service}"""); // TRACE%
      service.handleEvent(event).then((ServiceActionEvent responseEvent){
  