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/charts/lib/gallery_scaffold.dart

@@ -0,0 +1,62 @@
+// Copyright 2018 the Charts project authors. Please see the AUTHORS file
+// for details.
+//
+// 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';
+
+typedef Widget GalleryWidgetBuilder();
+
+/// Helper to build gallery.
+class GalleryScaffold extends StatefulWidget {
+  /// The widget used for leading in a [ListTile].
+  final Widget listTileIcon;
+  final String title;
+  final String subtitle;
+  final GalleryWidgetBuilder childBuilder;
+
+  GalleryScaffold(
+      {this.listTileIcon, this.title, this.subtitle, this.childBuilder});
+
+  /// Gets the gallery
+  Widget buildGalleryListTile(BuildContext context) => new ListTile(
+      leading: listTileIcon,
+      title: new Text(title),
+      subtitle: new Text(subtitle),
+      onTap: () {
+        Navigator.push(context, new MaterialPageRoute(builder: (_) => this));
+      });
+
+  @override
+  _GalleryScaffoldState createState() => new _GalleryScaffoldState();
+}
+
+class _GalleryScaffoldState extends State<GalleryScaffold> {
+  void _handleButtonPress() {
+    setState(() {});
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return new Scaffold(
+      appBar: new AppBar(title: new Text(widget.title)),
+      body: new Padding(
+          padding: const EdgeInsets.all(8.0),
+          child: new Column(children: <Widget>[
+            new SizedBox(height: 250.0, child: widget.childBuilder()),
+          ])),
+      floatingActionButton: new FloatingActionButton(
+          child: new Icon(Icons.refresh), onPressed: _handleButtonPress),
+    );
+  }
+}