use snackbar inside scaffold

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final key = new GlobalKey<ScaffoldState>();
    return new Scaffold(
      key: key,
      floatingActionButton: new Builder(
        builder: (BuildContext context) {
          return new FloatingActionButton(
            onPressed: () {
              key.currentState.showSnackBar(new SnackBar(
                content: new Text("Sending Message"),
              ));
            },
            tooltip: 'Increment',
            child: new Icon(Icons.add),
          );
        }
      ),
    );
  }
}