在main.dart中引入

import 'package:flutter/services.dart';

// wait until `setPreferredOrientations` is done and start the app
// this works.
void main() {
  SystemChrome.setPreferredOrientations([DeviceOrientation.portraitUp])
    .then((_) {
      runApp(new MyApp());
    });
}

setPreferredOrientations 会异步返回一个 Future 对象，我们需要等异步调用完成后再启动 App