Flutter how to programmatically exit the app

Below worked perfectly with me in both Android and iOS, I used exit(0) from dart:io

import 'dart:io';

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      body: new ... (...),
          floatingActionButton: new FloatingActionButton(
            onPressed: ()=> exit(0),
            tooltip: 'Close app',
            child: new Icon(Icons.close),
          ), 
    );
  }

UPDATE Jan 2019
Preferable solution is:

SystemChannels.platform.invokeMethod('SystemNavigator.pop');

As described here