Api

import 'dart:async';

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

class FBApi {
  static FirebaseAuth _auth = FirebaseAuth.instance;
  static GoogleSignIn _googleSignIn = GoogleSignIn();

  FirebaseUser firebaseUser;

  FBApi(FirebaseUser user) {
    this.firebaseUser = user;
  }

  static Future<FBApi> signInWithGoogle() async {
    final GoogleSignInAccount googleUser = await _googleSignIn.signIn();
    final GoogleSignInAuthentication googleAuth =
        await googleUser.authentication;
    try {
      final FirebaseUser user = await _auth.signInWithGoogle(
        accessToken: googleAuth.accessToken,
        idToken: googleAuth.idToken,
      );

      assert(user.email != null);
      assert(user.displayName != null);

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

      final FirebaseUser currentUser = await _auth.currentUser();
      assert(user.uid == currentUser.uid);

      return FBApi(user);
    } catch (error, stack) {
      print("Could not sign in with Google, here is the error $error");
      FlutterErrorDetails(exception: FlutterError(error), stack: stack);
    }
  }
}

app level gradle

import 'dart:async';
import 'dart:ui' show ImageFilter;

import 'package:firebase_example/Model/Api.dart';
import 'package:firebase_example/Ui/home.dart';
import 'package:flutter/material.dart';

class LoginPage extends StatefulWidget {
  @override
  LoginPageState createState() {
    return new LoginPageState();
  }
}

class LoginPageState extends State<LoginPage> {
  String _status;

  Future<bool> _loginUser() async {
    final api = await FBApi.signInWithGoogle();
    if (api != null) {
      setState(() {
        _status = 'Signed in with google';
      });
      return true;
    } else {
      print("fuuuuuuuuck");
      return false;
    }
  }

  @override
  void initState() {
    _status = 'Not Authenticated';
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        centerTitle: true,
        title: Text('Sign In'),
      ),
      body: Center(
        child: Stack(
          children: <Widget>[
            Center(
              child: ConstrainedBox(
                constraints: BoxConstraints.expand(
                  width: 250.0,
                  height: 250.0,
                ),
                child: Container(
                  color: Colors.cyan[100].withOpacity(0.7),
                ),
              ),
            ),
            Center(
              child: BackdropFilter(
                filter: ImageFilter.blur(
                  sigmaX: 5.0,
                  sigmaY: 5.0,
                ),
                child: Container(
                  width: 250.0,
                  height: 250.0,
                  color: Colors.grey[100].withOpacity(0.2),
                  child: Column(
                    mainAxisAlignment: MainAxisAlignment.spaceAround,
                    children: <Widget>[
                      Text(
                        "Login to the App!",
                        style: TextStyle(
                          color: Colors.black54,
                          fontSize: 30.0,
                          fontWeight: FontWeight.w600,
                        ),
                      ),
                      FlatButton(
                        color: Colors.black54,
                        onPressed: () async {
                          bool b = await _loginUser();

                          b
                              ? debugPrint("loged in with google") //Navigator.of(context).pushNamed('home-page')
                              : debugPrint("login fucked");
                          /*Scaffold.of(context).showSnackBar(
                                  SnackBar(
                                    content: Text('Wrong Email!'),
                                  ),
                                );*/
                        },
                        textColor: Colors.white.withOpacity(0.5),
                        child: Text('Sign In!'),
                      )
                    ],
                  ),
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

LoginPage

import 'dart:async';
import 'dart:ui' show ImageFilter;

import 'package:firebase_example/Model/Api.dart';
import 'package:firebase_example/Ui/home.dart';
import 'package:flutter/material.dart';

class LoginPage extends StatefulWidget {
  @override
  LoginPageState createState() {
    return new LoginPageState();
  }
}

class LoginPageState extends State<LoginPage> {
  String _status;

  Future<bool> _loginUser() async {
    final api = await FBApi.signInWithGoogle();
    if (api != null) {
      setState(() {
        _status = 'Signed in with google';
      });
      return true;
    } else {
      print("fuuuuuuuuck");
      return false;
    }
  }

  @override
  void initState() {
    _status = 'Not Authenticated';
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        centerTitle: true,
        title: Text('Sign In'),
      ),
      body: Center(
        child: Stack(
          children: <Widget>[
            Center(
              child: ConstrainedBox(
                constraints: BoxConstraints.expand(
                  width: 250.0,
                  height: 250.0,
                ),
                child: Container(
                  color: Colors.cyan[100].withOpacity(0.7),
                ),
              ),
            ),
            Center(
              child: BackdropFilter(
                filter: ImageFilter.blur(
                  sigmaX: 5.0,
                  sigmaY: 5.0,
                ),
                child: Container(
                  width: 250.0,
                  height: 250.0,
                  color: Colors.grey[100].withOpacity(0.2),
                  child: Column(
                    mainAxisAlignment: MainAxisAlignment.spaceAround,
                    children: <Widget>[
                      Text(
                        "Login to the App!",
                        style: TextStyle(
                          color: Colors.black54,
                          fontSize: 30.0,
                          fontWeight: FontWeight.w600,
                        ),
                      ),
                      FlatButton(
                        color: Colors.black54,
                        onPressed: () async {
                          bool b = await _loginUser();

                          b
                              ? debugPrint("loged in with google") //Navigator.of(context).pushNamed('home-page')
                              : debugPrint("login fucked");
                          /*Scaffold.of(context).showSnackBar(
                                  SnackBar(
                                    content: Text('Wrong Email!'),
                                  ),
                                );*/
                        },
                        textColor: Colors.white.withOpacity(0.5),
                        child: Text('Sign In!'),
                      )
                    ],
                  ),
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

project level gradle

buildscript {
    repositories {
        google()
        jcenter()
    }

    dependencies {
        classpath 'com.android.tools.build:gradle:3.0.1'
        classpath 'com.google.gms:google-services:4.0.0'
    }
}

allprojects {
    repositories {
        maven {
            url "https://maven.google.com"
        }
        google()
        jcenter()
    }
}

rootProject.buildDir = '../build'
subprojects {
    project.buildDir = "${rootProject.buildDir}/${project.name}"
}
subprojects {
    project.evaluationDependsOn(':app')
}

task clean(type: Delete) {
    delete rootProject.buildDir
}


subprojects {
    project.configurations.all {
        resolutionStrategy.eachDependency { details ->
            if (details.requested.group == 'com.android.support'
                    && !details.requested.name.contains('multidex') ) {
                details.useVersion "27.1.0"
            }
        }
    }
}