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/model/app_state_model.dart

@@ -0,0 +1,123 @@
+// 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:scoped_model/scoped_model.dart';
+
+import 'package:flutter_gallery/demo/shrine/model/product.dart';
+import 'package:flutter_gallery/demo/shrine/model/products_repository.dart';
+
+double _salesTaxRate = 0.06;
+double _shippingCostPerItem = 7.0;
+
+class AppStateModel extends Model {
+  // All the available products.
+  List<Product> _availableProducts;
+
+  // The currently selected category of products.
+  Category _selectedCategory = Category.all;
+
+  // The IDs and quantities of products currently in the cart.
+  final Map<int, int> _productsInCart = <int, int>{};
+
+  Map<int, int> get productsInCart => Map<int, int>.from(_productsInCart);
+
+  // Total number of items in the cart.
+  int get totalCartQuantity => _productsInCart.values.fold(0, (int v, int e) => v + e);
+
+  Category get selectedCategory => _selectedCategory;
+
+  // Totaled prices of the items in the cart.
+  double get subtotalCost {
+    return _productsInCart.keys
+      .map((int id) => _availableProducts[id].price * _productsInCart[id])
+      .fold(0.0, (double sum, int e) => sum + e);
+  }
+
+  // Total shipping cost for the items in the cart.
+  double get shippingCost {
+    return _shippingCostPerItem * _productsInCart.values.fold(0.0, (num sum, int e) => sum + e);
+  }
+
+  // Sales tax for the items in the cart
+  double get tax => subtotalCost * _salesTaxRate;
+
+  // Total cost to order everything in the cart.
+  double get totalCost => subtotalCost + shippingCost + tax;
+
+  // Returns a copy of the list of available products, filtered by category.
+  List<Product> getProducts() {
+    if (_availableProducts == null) {
+      return <Product>[];
+    }
+
+    if (_selectedCategory == Category.all) {
+      return List<Product>.from(_availableProducts);
+    } else {
+      return _availableProducts
+        .where((Product p) => p.category == _selectedCategory)
+        .toList();
+    }
+  }
+
+  // Adds a product to the cart.
+  void addProductToCart(int productId) {
+    if (!_productsInCart.containsKey(productId)) {
+      _productsInCart[productId] = 1;
+    } else {
+      _productsInCart[productId]++;
+    }
+
+    notifyListeners();
+  }
+
+  // Removes an item from the cart.
+  void removeItemFromCart(int productId) {
+    if (_productsInCart.containsKey(productId)) {
+      if (_productsInCart[productId] == 1) {
+        _productsInCart.remove(productId);
+      } else {
+        _productsInCart[productId]--;
+      }
+    }
+
+    notifyListeners();
+  }
+
+  // Returns the Product instance matching the provided id.
+  Product getProductById(int id) {
+    return _availableProducts.firstWhere((Product p) => p.id == id);
+  }
+
+  // Removes everything from the cart.
+  void clearCart() {
+    _productsInCart.clear();
+    notifyListeners();
+  }
+
+  // Loads the list of available products from the repo.
+  void loadProducts() {
+    _availableProducts = ProductsRepository.loadProducts(Category.all);
+    notifyListeners();
+  }
+
+  void setCategory(Category newCategory) {
+    _selectedCategory = newCategory;
+    notifyListeners();
+  }
+
+  @override
+  String toString() {
+    return 'AppStateModel(totalCost: $totalCost)';
+  }
+}