flutter_firebase_authentication_signInWithGoogle.dart

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

Future<FirebaseUser> signInWithGoogle() async {
  final GoogleSignInAccount googleUser = await googleSignIn.signIn();
  final GoogleSignInAuthentication googleAuth = await googleUser.authentication;

  final AuthCredential credential = GoogleAuthProvider.getCredential(
      idToken: googleAuth.idToken, accessToken: googleAuth.accessToken);
  AuthResult authResult = await _auth.signInWithCredential(credential);
  return authResult.user;
}