Update web/ samples to work with Flutter SDK (#134)

* add package:http dependency in dad_jokes

* add package:http dependency in filipino_cuisine

* don't build package:http demos until flutter/flutter#34858 is resolved

* update gallery

* update github_dataviz

* update particle_background

* don't build github_dataviz (uses package:http)

* update slide_puzzle

* update spinning_square

* update timeflow

* update vision_challenge

* update charts

* update dad_jokes

* update filipino cuisine

* ignore build output

* update timeflow and vision_challenge

* update slide_puzzle

* don't commit build/ directory

* move preview.png images to assets

* fix path url join

* update readme

* update web/readme.md

web/gallery/lib/demo/shrine/login.dart

@@ -0,0 +1,144 @@
+// Copyright 2018-present the Flutter authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import 'package:flutter/material.dart';
+
+import 'package:flutter_gallery/demo/shrine/colors.dart';
+
+class LoginPage extends StatefulWidget {
+  @override
+  _LoginPageState createState() => _LoginPageState();
+}
+
+class _LoginPageState extends State<LoginPage> {
+  final TextEditingController _usernameController = TextEditingController();
+  final TextEditingController _passwordController = TextEditingController();
+  static const ShapeDecoration _decoration =  ShapeDecoration(
+    shape: BeveledRectangleBorder(
+      side: BorderSide(color: kShrineBrown900, width: 0.5),
+      borderRadius: BorderRadius.all(Radius.circular(7.0)),
+    ),
+  );
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        elevation: 0.0,
+        backgroundColor: Colors.white,
+        brightness: Brightness.light,
+        leading: IconButton(
+          icon: const BackButtonIcon(),
+          tooltip: MaterialLocalizations.of(context).backButtonTooltip,
+          onPressed: () {
+            // The login screen is immediately displayed on top of the Shrine
+            // home screen using onGenerateRoute and so rootNavigator must be
+            // set to true in order to get out of Shrine completely.
+            Navigator.of(context, rootNavigator: true).pop();
+          },
+        ),
+      ),
+      body: SafeArea(
+        child: ListView(
+          padding: const EdgeInsets.symmetric(horizontal: 24.0),
+          children: <Widget>[
+            const SizedBox(height: 80.0),
+            Column(
+              children: <Widget>[
+                Image.asset('packages/shrine_images/diamond.png'),
+                const SizedBox(height: 16.0),
+                Text(
+                  'SHRINE',
+                  style: Theme.of(context).textTheme.headline,
+                ),
+              ],
+            ),
+            const SizedBox(height: 120.0),
+            PrimaryColorOverride(
+              color: kShrineBrown900,
+              child: Container(
+                decoration: _decoration,
+                child: TextField(
+                  controller: _usernameController,
+                  decoration: const InputDecoration(
+                    labelText: 'Username',
+                  ),
+                ),
+              ),
+            ),
+            const SizedBox(height: 12.0),
+            PrimaryColorOverride(
+              color: kShrineBrown900,
+              child: Container(
+                decoration: _decoration,
+                child: TextField(
+                  controller: _passwordController,
+                  decoration: const InputDecoration(
+                    labelText: 'Password',
+                  ),
+                ),
+              ),
+            ),
+            Wrap(
+              children: <Widget>[
+                ButtonBar(
+                  children: <Widget>[
+                    FlatButton(
+                      child: const Text('CANCEL'),
+                      shape: const BeveledRectangleBorder(
+                        borderRadius: BorderRadius.all(Radius.circular(7.0)),
+                      ),
+                      onPressed: () {
+                        // The login screen is immediately displayed on top of
+                        // the Shrine home screen using onGenerateRoute and so
+                        // rootNavigator must be set to true in order to get out
+                        // of Shrine completely.
+                        Navigator.of(context, rootNavigator: true).pop();
+                      },
+                    ),
+                    RaisedButton(
+                      child: const Text('NEXT'),
+                      elevation: 8.0,
+                      shape: const BeveledRectangleBorder(
+                        borderRadius: BorderRadius.all(Radius.circular(7.0)),
+                      ),
+                      onPressed: () {
+                        Navigator.pop(context);
+                      },
+                    ),
+                  ],
+                ),
+              ],
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}
+
+class PrimaryColorOverride extends StatelessWidget {
+  const PrimaryColorOverride({Key key, this.color, this.child}) : super(key: key);
+
+  final Color color;
+  final Widget child;
+
+  @override
+  Widget build(BuildContext context) {
+    return Theme(
+      child: child,
+      data: Theme.of(context).copyWith(primaryColor: color),
+    );
+  }
+}