Added a new example for google signin.

google_signin/lib/home.dart

@@ -0,0 +1,40 @@
+import 'package:flutter/material.dart';
+import 'package:meta/meta.dart';
+import 'package:flutter/foundation.dart';
+
+class Home extends StatelessWidget {
+  Home(
+      {Key key,
+      @required this.onSignin,
+      @required this.onLogout,
+      @required this.showLoading})
+      : super(key: key);
+
+  final VoidCallback onSignin;
+  final VoidCallback onLogout;
+  bool showLoading = false;
+
+  @override
+  Widget build(BuildContext context) {
+    return new Scaffold(
+      appBar: new AppBar(title: new Text("Sign In")),
+      body: new Container(
+          padding: const EdgeInsets.all(20.0),
+          child: new Center(
+            child: new Column(
+              mainAxisAlignment: MainAxisAlignment.center,
+              children: <Widget>[
+                showLoading
+                    ? new CircularProgressIndicator()
+                    : new RaisedButton(
+                        onPressed: this.onSignin,
+                        child: new Text("Sign In"),
+                        color: Colors.lightBlueAccent,
+                      ),
+                //new RaisedButton(onPressed: this.onLogout, child: new Text("Logout"), color: Colors.amberAccent),
+              ],
+            ),
+          )),
+    );
+  }
+}

google_signin/lib/main.dart

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+import 'package:flutter/material.dart';
+import 'package:google_sign_in/google_sign_in.dart';
+import 'package:firebase_auth/firebase_auth.dart';
+import 'home.dart';
+import 'user.dart';
+import 'dart:async';
+
+void main() {
+  runApp(new App());
+}
+
+class App extends StatefulWidget {
+  AppState createState() => new AppState();
+}
+
+class AppState extends State<App> {
+  String _username = "";
+  Widget currentPage;
+  GoogleSignIn googleSignIn;
+  Widget userPage;
+  @override
+  void initState() {
+    super.initState();
+    userPage = new Home(
+      onSignin: () {
+        _signin();
+        print("Sign");
+      },
+      onLogout: _logout,
+      showLoading: false,
+    );
+  }
+
+  Future<FirebaseUser> _signin() async {
+    setState(() {
+      userPage = new Home(onSignin: null, onLogout: _logout, showLoading: true);
+    });
+    FirebaseAuth _auth = FirebaseAuth.instance;
+    try {
+      googleSignIn = new GoogleSignIn();
+      GoogleSignInAccount googleSignInAccount = await googleSignIn.signIn();
+      GoogleSignInAuthentication gauth =
+          await googleSignInAccount.authentication;
+      FirebaseUser user = await _auth.signInWithGoogle(
+        accessToken: gauth.accessToken,
+        idToken: gauth.idToken,
+      );
+
+      setState(() {
+        _username = user.displayName;
+        userPage = new User(
+          onLogout: _logout,
+          user: user,
+        );
+      });
+
+      return user;
+    } catch (e) {
+      print(e.toString());
+    }
+    return null;
+  }
+
+  void _logout() async {
+    await googleSignIn.signOut();
+    setState(() {
+      userPage = new Home(
+        onSignin: () {
+          _signin();
+          print("Sign");
+        },
+        onLogout: _logout,
+        showLoading: false,
+      );
+    });
+
+    print("Logged Out");
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return new MaterialApp(
+      home: userPage,
+    );
+  }
+}

google_signin/lib/user.dart

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+import 'package:flutter/material.dart';
+import 'package:meta/meta.dart';
+import 'package:flutter/foundation.dart';
+import 'package:firebase_auth/firebase_auth.dart';
+
+class User extends StatelessWidget {
+  User({Key key, @required this.onLogout, @required this.user})
+      : super(key: key);
+
+  VoidCallback onLogout;
+  String username;
+  FirebaseUser user;
+
+  @override
+  Widget build(BuildContext context) {
+    return new Scaffold(
+      appBar: new AppBar(
+        title: new Text("Welcome"),
+        actions: <Widget>[
+          new IconButton(
+              icon: new Icon(Icons.exit_to_app), onPressed: this.onLogout)
+        ],
+      ),
+      body: new Container(
+          padding: const EdgeInsets.all(20.0),
+          child: new Center(
+              child: new Column(
+            mainAxisAlignment: MainAxisAlignment.center,
+            children: <Widget>[
+              new Image.network(user.photoUrl),
+              new Text(
+                user.displayName,
+                textScaleFactor: 1.5,
+              ),
+            ],
+          ))),
+    );
+  }
+}