WillPopScope

Widget build(BuildContext context) {
    return WillPopScope(
      onWillPop: _onExit,
      child: Scaffold();

Future<bool> _onExit() async {
    return showDialog(
        context: context,
        builder: (context) => AlertDialog(
              title: Text("Exit"),
              content: Text("Do you want to exit?"),
              actions: <Widget>[
                RaisedButton(
                  child: Text(
                    "Nope",
                  ),
                  onPressed: () => Navigator.of(context).pop(),
                ),
                FlatButton(
                    child: Text(
                      "Yes",
                      style: TextStyle(color: Colors.red[200]),
                    ),
                    onPressed: () => exit(0)),
              ],
            ));
  }
}