//Modify the ChatMessage class definition in main.dart.

...

new Expanded(                                               //new
  child: new Column(                                   //modified
    crossAxisAlignment: CrossAxisAlignment.start,
    children: <Widget>[
      new Text(_name, style: Theme.of(context).textTheme.subhead),
      new Container(
        margin: const EdgeInsets.only(top: 5.0),
        child: new Text(text),
      ),
    ],
  ),
),                                                          //new

...