anchor.diff:    void schedule(SendPort port) {
       assert(_isolate != null);
       if (_queue.isEmpty()) {
  -      port.send({ "type": _FileOperationIsolate.EXIT });
  +      port.send(new _ExitOperation());
         _isolate = null;
       } else {
         port.send(_queue.removeFirst());
  
positive.code:     _asyncUsed = true;
      var handler =
          (_existsHandler != null) ? _existsHandler : (result) => null;
      Map params = {
        "type": _FileOperationIsolate.EXISTS,
        "name": _name
      };
      _scheduler.enqueue(params, (result, ignored) { _existsHandler(result); });
    }
  
    bool existsSync() {
  
negative.code:         _errorHandler("readByte failed");
        }
      };
      var operation = new _ReadByteOperation(_id);
      _scheduler.enqueue(operation, handleReadByteResult);
    }
  
    int readByteSync() {
  