// Add the following code to main.dart.

import 'package:flutter/cupertino.dart';                      //new

// Modify the _buildTextComposer method.

new Container(
   margin: new EdgeInsets.symmetric(horizontal: 4.0),
   child: Theme.of(context).platform == TargetPlatform.iOS ?  //modified
   new CupertinoButton(                                       //new
     child: new Text("Send"),                                 //new
     onPressed: _isComposing                                  //new
         ? () =>  _handleSubmitted(_textController.text)      //new
         : null,) :                                           //new
   new IconButton(                                            //modified
       icon: new Icon(Icons.send),
       onPressed: _isComposing ?
           () =>  _handleSubmitted(_textController.text) : null,
       )
   ),