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/cupertino/cupertino_buttons_demo.dart

@@ -0,0 +1,89 @@
+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/cupertino.dart';
+
+import '../../gallery/demo.dart';
+
+class CupertinoButtonsDemo extends StatefulWidget {
+  static const String routeName = '/cupertino/buttons';
+
+  @override
+  _CupertinoButtonDemoState createState() => _CupertinoButtonDemoState();
+}
+
+class _CupertinoButtonDemoState extends State<CupertinoButtonsDemo> {
+  int _pressedCount = 0;
+
+  @override
+  Widget build(BuildContext context) {
+    return CupertinoPageScaffold(
+      navigationBar: CupertinoNavigationBar(
+        middle: const Text('Buttons'),
+        // We're specifying a back label here because the previous page is a
+        // Material page. CupertinoPageRoutes could auto-populate these back
+        // labels.
+        previousPageTitle: 'Cupertino',
+        trailing: CupertinoDemoDocumentationButton(CupertinoButtonsDemo.routeName),
+      ),
+      child: DefaultTextStyle(
+        style: CupertinoTheme.of(context).textTheme.textStyle,
+        child: SafeArea(
+          child: Column(
+            children: <Widget>[
+              const Padding(
+                padding: EdgeInsets.all(16.0),
+                child: Text(
+                  'iOS themed buttons are flat. They can have borders or backgrounds but '
+                  'only when necessary.'
+                ),
+              ),
+              Expanded(
+                child: Column(
+                  mainAxisAlignment: MainAxisAlignment.center,
+                  children: <Widget> [
+                    Text(_pressedCount > 0
+                        ? 'Button pressed $_pressedCount time${_pressedCount == 1 ? "" : "s"}'
+                        : ' '),
+                    const Padding(padding: EdgeInsets.all(12.0)),
+                    Align(
+                      alignment: const Alignment(0.0, -0.2),
+                      child: Row(
+                        mainAxisSize: MainAxisSize.min,
+                        children: <Widget>[
+                          CupertinoButton(
+                            child: const Text('Cupertino Button'),
+                            onPressed: () {
+                              setState(() { _pressedCount += 1; });
+                            },
+                          ),
+                          const CupertinoButton(
+                            child: Text('Disabled'),
+                            onPressed: null,
+                          ),
+                        ],
+                      ),
+                    ),
+                    const Padding(padding: EdgeInsets.all(12.0)),
+                    CupertinoButton.filled(
+                      child: const Text('With Background'),
+                      onPressed: () {
+                        setState(() { _pressedCount += 1; });
+                      },
+                    ),
+                    const Padding(padding: EdgeInsets.all(12.0)),
+                    const CupertinoButton.filled(
+                      child: Text('Disabled'),
+                      onPressed: null,
+                    ),
+                  ],
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}