gistfile1.dart

class FutureGroup {
  Completer _completer = new Completer();
  List<Future> _futures = new List<Future>();
  
  FutureGroup();
  
  bool add(Future future) {
    if(!_futures.contains(future)) {
      _futures.add(future);
      return true;
    }
    return false;
  }
  
  Future wait() {
    Future.wait(_futures).then((result) {
      _completer.complete();
    })..catchError((err) => _completer.completeError(err));
    
    return _completer.future;
  }
}