anchor.diff:    
     String _currentId;
     Map<String, Function> _pendingCallbacks;
  +  
  +  final List<String> _callbacks = <String>[];
   
     //---------------------------------
     //
  
positive.code:   }
    
    void addCallback(String callbackMethod, void callbackHandler(String jsonData)) {
      _injectScript(callbackMethod);
      
      if (isInitialized) context['_iop_${_currentId}_$callbackMethod'] = (String value) => callbackHandler(value);
      else {
        if (_pendingCallbacks == null) _pendingCallbacks = <String, Function>{};
        
  
negative.code:     }
    }
    
    void send(String exposedFlashMethod, String value, {String callbackMethod, void callbackHandler(String jsonData)}) {
      final bool expectsCallback = ((callbackMethod != null) && (callbackHandler != null));
      
      if (expectsCallback) addCallback(
          callbackMethod,
          callbackHandler
      );
      
      try {
        context.callMethod('_iop_${_currentId}_writeExternal', [exposedFlashMethod, value]);
      } catch (error) {
  