An example of dealing with auth methods in Flutter with FB and Google sign in

auth.dart

import 'dart:async';

import 'package:appname/services/firestore.dart';
import 'package:appname/utils/get_status_code_error.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter_facebook_login/flutter_facebook_login.dart';
import 'package:google_sign_in/google_sign_in.dart';

GoogleSignIn _googleSignIn = GoogleSignIn(scopes: ['email']);

final auth = FirebaseAuth.instance;
final _facebookLogin = FacebookLogin();

Future<void> signOut() async {
  await Future.wait([
    _googleSignIn.signOut(),
    auth.signOut(),
    _facebookLogin.logOut(),
  ]);
}

Future<void> facebookSignIn() async {
  try {
    final FacebookLoginResult result = await _facebookLogin.logIn(['email']);

    if (result.status == FacebookLoginStatus.cancelledByUser)
      throw 'User Canceled Permissions';
    if (result.status == FacebookLoginStatus.error)
      throw 'Error Authenticating User';

    final accessToken = result.accessToken.token;

    final AuthCredential credential = FacebookAuthProvider.getCredential(
      accessToken: accessToken,
    );

    await auth.signInWithCredential(credential).then((authResult) {
      updateUserData(authResult.user);
    });
  } catch (error) {
    throw _errorToHumanReadable(error.toString());
  }
}

Future<void> googleSignIn() async {
  try {
    GoogleSignInAccount googleUser = await _googleSignIn.signIn();
    if (googleUser == null) throw 'User Canceled Permissions';
    GoogleSignInAuthentication googleAuth = await googleUser.authentication;

    final signInMethods =
        await auth.fetchSignInMethodsForEmail(email: googleUser.email);

    if (signInMethods.length != 0 && !signInMethods.contains('google.com'))
      throw 'An account already exists with the same email address but different sign-in credentials. Sign in using a provider associated with this email address.';

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

    await auth.signInWithCredential(credential).then((authResult) {
      updateUserData(authResult.user);
    });
  } catch (error) {
    throw _errorToHumanReadable(error.toString());
  }
}

Future<void> anonymousSignIn() async {
  try {
    await auth.signInAnonymously();
  } catch (error) {
    throw _errorToHumanReadable(error.toString());
  }
}

///
/// Maps an error string to typical `developer` readable error messages.
/// See [getStatusCodeError].
String _errorToHumanReadable(String errorMessage) {
  final exp = RegExp(r':\s([0-9]+):');
  final match = exp.firstMatch(errorMessage);

  if (match == null) {
    return errorMessage;
  } else {
    final group = match.group(1);
    final statusCode = int.tryParse(group) ?? 99;

    return getStatusCodeError(statusCode: statusCode);
  }
}