如何在firebase和flutter中使用OAuth凭据注册和登录？

dependencies:
  firebase_auth: ^0.20.1
  google_sign_in: ^4.5.1

apply plugin: 'com.google.gms.google-services'

import 'package:firebase_auth/firebase_auth.dart';
import 'package:google_sign_in/google_sign_in.dart';

final FirebaseAuth _auth = FirebaseAuth.instance;
final GoogleSignIn _googleSignIn = GoogleSignIn();

Future<UserCredential> signInWithGoogle() async {
  final GoogleSignInAccount googleSignInAccount = await _googleSignIn.signIn();
  final GoogleSignInAuthentication googleSignInAuthentication =
      await googleSignInAccount.authentication;

  final AuthCredential credential = GoogleAuthProvider.credential(
    accessToken: googleSignInAuthentication.accessToken,
    idToken: googleSignInAuthentication.idToken,
  );

  final UserCredential authResult = await _auth.signInWithCredential(credential);
  final User user = authResult.user;

  assert(!user.isAnonymous);
  assert(await user.getIdToken() != null);

  final User currentUser = _auth.currentUser;
  assert(user.uid == currentUser.uid);

  return authResult;
}

void signOutGoogle() async {
  await _googleSignIn.signOut();
  print("User Signed Out");
}

import 'auth.dart';

class LoginPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Login'),
      ),
      body: Center(
        child: RaisedButton(
          child: Text('Sign in with Google'),
          onPressed: () {
            signInWithGoogle().then((userCredential) {
              // 登录成功后的处理逻辑
            }).catchError((error) {
              // 登录失败后的处理逻辑
            });
          },
        ),
      ),
    );
  }
}

import 'auth.dart';

class RegisterPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Register'),
      ),
      body: Center(
        child: RaisedButton(
          child: Text('Register with Google'),
          onPressed: () {
            signInWithGoogle().then((userCredential) {
              // 注册成功后的处理逻辑
            }).catchError((error) {
              // 注册失败后的处理逻辑
            });
          },
        ),
      ),
    );
  }
}