如何在flutter中使用firebase登录google时检查用户是新用户还是现有用户？

FirebaseUser user = await FirebaseAuth.instance.currentUser();

bool isNewUser = user.isAnonymous;

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

class LoginScreen extends StatefulWidget {
  @override
  _LoginScreenState createState() => _LoginScreenState();
}

class _LoginScreenState extends State<LoginScreen> {
  FirebaseAuth _auth = FirebaseAuth.instance;

  Future<void> _signInWithGoogle() async {
    try {
      GoogleSignInAccount googleUser = await GoogleSignIn().signIn();
      GoogleSignInAuthentication googleAuth = await googleUser.authentication;

      AuthCredential credential = GoogleAuthProvider.getCredential(
        accessToken: googleAuth.accessToken,
        idToken: googleAuth.idToken,
      );

      AuthResult authResult = await _auth.signInWithCredential(credential);
      FirebaseUser user = authResult.user;

      // 判断用户是否为新用户
      bool isNewUser = user.isAnonymous;
      if (isNewUser) {
        // 处理新用户的注册流程
        // ...
      } else {
        // 处理已注册用户的登录流程
        // ...
      }
    } catch (e) {
      // 处理登录异常
      print(e);
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Login'),
      ),
      body: Center(
        child: RaisedButton(
          child: Text('Sign in with Google'),
          onPressed: _signInWithGoogle,
        ),
      ),
    );
  }
}