Flutter/モジュール分割

lib/main.dart

import 'package:flutter/material.dart';
import 'gallery/app.dart';
void main() {
  runApp(const GalleryApp());
}

gallery/app.dart

class GalleryApp extends StatefulWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Hello Wolrd',
      home: new Scaffold(
        body: new Center(
          child: new Text('Hello World'),
        ),
      ),
    );
  }
}