anchor.diff:    }
     _asyncLevel--;
     if (_asyncLevel == 0) {
  -    _port.close();
  -    _port = null;
       print('unittest-suite-success');
     }
   }
  
positive.code: 
  library async_helper;
  
  // TODO(kustermann): This is problematic because we rely on a working
  // 'dart:isolate' (i.e. it is in particular problematic with dart2js).
  // It would be nice if we could use a different mechanism for different
  // runtimes.
  import 'dart:isolate';
  
  bool _initialized = false;
  ReceivePort _port = null;
  int _asyncLevel = 0;
  
  Exception _buildException(String msg) {
  
negative.code:   if (!_initialized) {
      print('unittest-suite-wait-for-done');
      _initialized = true;
    }
    _asyncLevel += count;
  }
  