void main() => runApp(App());

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: Padding(
            padding: const EdgeInsets.all(8.0),
            child: Test(),
          ),
        ),
      ),
    );
  }
}

class Test extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Row(
      children: <Widget>[
        Container(width: 50, height: 50, color: Colors.red),
        Spacer(),
        Container(width: 50, height: 50, color: Colors.yellow),
        Spacer(),
      ],
    );
  }
}