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/home.dart

@@ -0,0 +1,57 @@
+// 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:scoped_model/scoped_model.dart';
+
+import 'package:flutter_gallery/demo/shrine/backdrop.dart';
+import 'package:flutter_gallery/demo/shrine/expanding_bottom_sheet.dart';
+import 'package:flutter_gallery/demo/shrine/model/app_state_model.dart';
+import 'package:flutter_gallery/demo/shrine/model/product.dart';
+import 'package:flutter_gallery/demo/shrine/supplemental/asymmetric_view.dart';
+
+class ProductPage extends StatelessWidget {
+  const ProductPage({this.category = Category.all});
+
+  final Category category;
+
+  @override
+  Widget build(BuildContext context) {
+    return ScopedModelDescendant<AppStateModel>(
+      builder: (BuildContext context, Widget child, AppStateModel model) {
+        return AsymmetricView(products: model.getProducts());
+      });
+  }
+}
+
+class HomePage extends StatelessWidget {
+  const HomePage({
+    this.expandingBottomSheet,
+    this.backdrop,
+    Key key,
+  }) : super(key: key);
+
+  final ExpandingBottomSheet expandingBottomSheet;
+  final Backdrop backdrop;
+
+  @override
+  Widget build(BuildContext context) {
+    return Stack(
+      children: <Widget>[
+        backdrop,
+        Align(child: expandingBottomSheet, alignment: Alignment.bottomRight),
+      ],
+    );
+  }
+}