anchor.diff:        'text': text,
       };
       _firebase.push().set(message);
  -  }
  -
  -  Widget _buildMessage(int index) {
  -    Map<String, String> message = _messages[index];
  -    return new Container(
  -      key: new ValueKey(index),
  -      margin: const EdgeDims.all(3.0),
  -      child: new Center(
  -        child: new Text("${message['name']}: ${message['text']}")
  -      )
  -    );
     }
   
     GlobalKey _messageKey = new GlobalKey();
  
positive.code:   State createState() => new FirechatAppState();
  }
  
  class FirechatAppState extends State {
    Firebase _firebase;
    List<Map<String, String>> _messages;
  
negative.code:             new Flexible(
                child: new Block(
                  scrollAnchor: ViewportAnchor.end,
                  children: _messages.map((m) => new ChatMessage(m)).toList()
                )
              ),
              _buildTextComposer(),
  