Flutter退出App的方法一般有两种

①SystemNavigator.pop 推荐

onTap: () async {
              await pop();
            },

static Future<void> pop() async {
    await SystemChannels.platform.invokeMethod('SystemNavigator.pop');
  }

别忘了引用

import 'package:flutter/services.dart';

原文参考： https://docs.flutter.io/flutter/services/SystemNavigator/pop.html

②exit(0)

onPressed: ()=> exit(0),

原文参考： https://docs.flutter.io/flutter/dart-io/exit.html