Create Firebase auth services to utilize in Flutter

frontend/memoree_client/lib/app/models/user.dart

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+import 'package:meta/meta.dart';
+
+@immutable
+class User {
+  final String uid;
+  final String email;
+  final String photoUrl;
+  final String displayName;
+
+  const User({
+    @required this.uid,
+    this.email,
+    this.photoUrl,
+    this.displayName
+  });
+}

frontend/memoree_client/lib/app/services/firebase_auth.dart

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+import 'package:firebase_auth/firebase_auth.dart' as auth;
+import 'package:firebase_core/firebase_core.dart';
+import 'package:google_sign_in/google_sign_in.dart';
+import 'package:memoree_client/app/models/user.dart';
+
+class FirebaseAuthService {
+  final auth.FirebaseAuth _firebaseAuth;
+  final GoogleSignIn _googleSignIn;
+
+  FirebaseAuthService({auth.FirebaseAuth firebaseAuth, GoogleSignIn googleSignin})
+      : _firebaseAuth = firebaseAuth ?? auth.FirebaseAuth.instance,
+        _googleSignIn = googleSignin ?? GoogleSignIn();
+
+  User _userFromFirebase(auth.User user) {
+    if (user == null) {
+      return null;
+    }
+    return User(
+      uid: user.uid,
+      email: user.email,
+      displayName: user.displayName,
+      photoUrl: user.photoURL,
+    );
+  }
+
+  Stream<User> get onAuthStateChanged {
+    return _firebaseAuth.authStateChanges().map(_userFromFirebase);
+  }
+
+  Future<User> signInWithGoogle() async {
+    await Firebase.initializeApp();
+
+    final googleUser = await _googleSignIn.signIn();
+    final googleAuth = await googleUser.authentication;
+    final credential = auth.GoogleAuthProvider.credential(
+      accessToken: googleAuth.accessToken,
+      idToken: googleAuth.idToken,
+    );
+    final authResult = await _firebaseAuth.signInWithCredential(credential);
+    return _userFromFirebase(authResult.user);
+  }
+
+  Future<void> signOut() async {
+    await _googleSignIn.disconnect();
+    return _firebaseAuth.signOut();
+  }
+
+  Future<User> currentUser() async {
+    final user = _firebaseAuth.currentUser;
+    return _userFromFirebase(user);
+  }
+}