anchor.diff:  }
   
   class D {
  -  IOSink init(filename) {
  +  IOSink init(String filename) {
       IOSink _sinkF = new File(filename).openWrite(); // OK
       return _sinkF;
     }
  
positive.code: 
  class B1 {
    Socket _socketB1;
    Future init(filename) async {
      _socketB1 = await Socket.connect(null /*address*/, 1234); // OK
    }
  
    void dispose(filename) {
      _socketB1.destroy();
    }
  }
  
negative.code:       ..add(null)
        ..close();
    }
  }
  