▶ 运行代码import "package:flutter/material.dart";
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
        return MaterialApp(
            home: Scaffold(
                appBar: AppBar(title: Text("Flutter Demo")),
                body: Center(child: Text("Hello Flutter!"))
            )
        );
    }
}