Example code of how to sign in with google.

GoogleSignIn.dart

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


void _signInWithGoogle() async {
    final GoogleSignInAccount googleUser = await _googleSignIn.signIn();
    final GoogleSignInAuthentication googleAuth =
        await googleUser.authentication;
    final AuthCredential credential = GoogleAuthProvider.getCredential(
      accessToken: googleAuth.accessToken,
      idToken: googleAuth.idToken,
    );
    final FirebaseUser user = await _auth.signInWithCredential(credential);
    assert(user.email != null);
    assert(user.displayName != null);
    assert(!user.isAnonymous);
    assert(await user.getIdToken() != null);

    final FirebaseUser currentUser = await _auth.currentUser();
    assert(user.uid == currentUser.uid);
    setState(() {
      if (user != null) {
        _success = true;
        _userID = user.uid;
        _email = user.email;
        _name = user.displayName;
        _ppic = user.photoUrl;
        return _userID;
      } else {
        _success = false;
      }
    });
  }