Create a new Dart library.
    For example, create a new DeferredBox widget that
    can be downloaded at runtime.
    This widget can be of any complexity but,
    for the purposes of this guide,
    create a simple box as a stand-in.
    To create a simple blue box widget,
    create box.dart with the following contents:

    
// box.dart
import 'package:flutter/material.dart';

/// A simple blue 30x30 box.
class DeferredBox extends StatelessWidget {
  const DeferredBox({super.key});

  @override
  Widget build(BuildContext context) {
    return Container(
      height: 30,
      width: 30,
      color: Colors.blue,
    );
  }
}