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/app_config.dart

@@ -0,0 +1,40 @@
+// 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';
+
+/// A particular configuration of the app.
+class AppConfig {
+  final String appName;
+  final String appLink;
+  final ThemeData theme;
+  final bool showPerformanceOverlay;
+
+  AppConfig(
+      {this.appName, this.appLink, this.theme, this.showPerformanceOverlay});
+}
+
+/// The default configuration of the app.
+AppConfig get defaultConfig {
+  return new AppConfig(
+    appName: 'Charts Gallery',
+    appLink: '',
+    theme: new ThemeData(
+      brightness: Brightness.light,
+      primarySwatch: Colors.lightBlue,
+    ),
+    showPerformanceOverlay: false,
+  );
+}