anchor.diff:              new Flexible(
                 child: new Block(
                   scrollAnchor: ViewportAnchor.end,
  -                children: new Iterable.generate(_messages.length)
  -                                      .map((index) => _buildMessage(index))
  -                                      .toList()
  +                children: _messages.map((m) => new ChatMessage(m)).toList()
                 )
               ),
               _buildTextComposer(),
  
positive.code:   State createState() => new FirechatAppState();
  }
  
  class FirechatAppState extends State {
    Firebase _firebase;
    List<Map<String, String>> _messages;
  
negative.code:     _user = "Guest${new math.Random().nextInt(1000)}";
      _currentMessage = '';
      _firebase.onChildAdded.listen((Event event) {
        setState(() => _messages.add(event.snapshot.val()));
      });
      _messages = <Map<String, String>>[];
      super.initState();
  