AndroidX Firebase/Google Auth

        final FirebaseAuth _auth = FirebaseAuth.instance;

        // GoogleSignIn is a specific sign in class.
        final GoogleSignIn _googleSignIn = GoogleSignIn();

        // Try to sign in the user.
        // This method will either log in a user that your Firebase
        // is aware of, or it will prompt the user to log in
        // if its the first time.
        //
        GoogleSignInAccount googleUser = await _googleSignIn.signIn();
        GoogleSignInAuthentication googleAuth = await googleUser.authentication;
        AuthCredential authCredential = GoogleAuthProvider.getCredential(
            idToken: googleAuth.idToken, accessToken: googleAuth.accessToken);
        // After checking for authentication,
        // We wil actually sign in the user
        // using the token that firebase.
        user = await _auth.signInWithCredential(authCredential);