anchor.diff:   */
   class FutureNotCompleteException implements Exception {
     FutureNotCompleteException() {}
  +  String toString() => "Exception: future has not been completed";
   }
   
   /**
  
positive.code:  */
  class FutureAlreadyCompleteException implements Exception {
    FutureAlreadyCompleteException() {}
  }
  
  
  
negative.code:   void then(void onComplete(T value)) {
      if (hasValue) {
        onComplete(value);
      } else if (!isComplete) {
        _listeners.add(onComplete);
      } else if (!_exceptionHandled) {
        throw _exception;
      }
    }
  
  