FlutterでFirebaseを連携したい

前提・実現したいこと

FlutterでFirebaseを連携したいのですが、

import 'package:firebase_auth/firebase_auth.dart';
import 'package:google_sign_in/google_sign_in.dart';
dartファイル内の上記2ファイルがエラーになってしまいます。

以下のサイトを参考に実装しました。
参考サイト

ご教授お願い致します。

発生している問題・エラーメッセージ

Target of URI doesn't exist: package:firebase_auth/firebase_auth.dart


Target of URI doesn't exist: package:google_sign_in/google_sign_in.dart

該当のソースコード

android/build.gradleに追記

dependencies {
        classpath 'com.google.gms:google-services:3.2.1'
        classpath 'com.android.tools.build:gradle:3.5.0'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
    }

android/app/build.gradleに追記

apply plugin: 'com.google.gms.google-services'

pubspec.yaml

dependencies:
  flutter:
    sdk: flutter
  firebase_core: ^0.2.5  # add dependency for Firebase Core
  firebase_auth: ^0.6.6
  google_sign_in: ^3.2.4

dart
1import 'package:firebase_auth/firebase_auth.dart';
2import 'package:flutter/material.dart';
3import 'package:google_sign_in/google_sign_in.dart';
4
5void main() => runApp(MyApp());
6
7class MyApp extends StatelessWidget {
8  @override
9  Widget build(BuildContext context) {
10    return MaterialApp(
11      title: 'Flutter × firebase Demo',
12      theme: ThemeData(
13        primarySwatch: Colors.blue,
14      ),
15      home: MyHomePage(title: 'Flutter × firebase Demor'),
16    );
17  }
18}
19
20class MyHomePage extends StatefulWidget {
21  MyHomePage({Key key, this.title}) : super(key: key);
22
23  final String title;
24
25  @override
26  _MyHomePageState createState() => _MyHomePageState();
27}
28
29class _MyHomePageState extends State<MyHomePage> {
30  FirebaseUser _user;
31
32  void _setUser(FirebaseUser user) {
33    setState(() {
34      _user = user;
35    });
36  }
37
38  final GoogleSignIn _googleSignIn = GoogleSignIn();
39  final FirebaseAuth _auth = FirebaseAuth.instance;
40
41  Future<FirebaseUser> _handleSignIn() async {
42    GoogleSignInAccount googleUser = await _googleSignIn.signIn();
43    GoogleSignInAuthentication googleAuth = await googleUser.authentication;
44    FirebaseUser user = await _auth.signInWithGoogle(
45      accessToken: googleAuth.accessToken,
46      idToken: googleAuth.idToken,
47    );
48    print("signed in " + user.displayName);
49    return user;
50  }
51
52  @override
53  Widget build(BuildContext context) {
54    return Scaffold(
55        appBar: AppBar(
56          title: Text(widget.title),
57        ),
58        body: _user == null ? googleAuthBtn() : chat());
59  }
60
61  Widget googleAuthBtn() {
62    return Center(
63      child: Column(
64        mainAxisAlignment: MainAxisAlignment.center,
65        children: <Widget>[
66          FlatButton(
67            padding: EdgeInsets.all(20.0),
68            color: Colors.grey,
69            onPressed: () {
70              _handleSignIn()
71                  .then((FirebaseUser user) => _setUser(user))
72                  .catchError((e) => print(e));
73            },
74            child: Text('google認証'),
75          ),
76        ],
77      ),
78    );
79  }
80
81  Widget chat() {
82    return Center(
83      child: Column(
84        mainAxisAlignment: MainAxisAlignment.center,
85        children: <Widget>[Text('chat')],
86      ),
87    );
88  }
89}
90