// Modify the _handleSubmittted method definition as follows.

void _handleSubmitted(String text) {
  _textController.clear();
  ChatMessage message = new ChatMessage(
    text: text,
    animationController: new AnimationController(                  //new
      duration: new Duration(milliseconds: 700),                   //new
      vsync: this,                                                 //new
    ),                                                             //new
  );                                                               //new
  setState(() {
    _messages.insert(0, message);
  });
  message.animationController.forward();                           //new
}