component.dart

class Container extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return MyButton(title: 'Titulo do button');
  }
}

class MyButton extends StatelessWidget {
  String title;

  // Esse é o construtor, prazer
  Button({this.title});

  @override
  Widget build(BuildContext context) {
    return Button(title);
  }
}