Add provider_shopper (#87)

This is the code for the sample at https://flutter.dev/docs/development/data-and-backend/state-mgmt/simple.

provider_shopper/lib/models/cart.dart

@@ -0,0 +1,38 @@
+// Copyright 2019 The Flutter team. 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/foundation.dart';
+import 'package:provider_shopper/models/catalog.dart';
+
+class CartModel extends ChangeNotifier {
+  /// The current catalog. Used to construct items from numeric ids.
+  final CatalogModel _catalog;
+
+  /// Internal, private state of the cart. Stores the ids of each item.
+  final List<int> _itemIds;
+
+  /// Construct a CartModel instance that is backed by a [CatalogModel] and
+  /// an optional previous state of the cart.
+  ///
+  /// If [previous] is not `null`, it's items are copied to the newly
+  /// constructed instance.
+  CartModel(this._catalog, CartModel previous)
+      : assert(_catalog != null),
+        _itemIds = previous?._itemIds ?? [];
+
+  /// List of items in the cart.
+  List<Item> get items => _itemIds.map((id) => _catalog.getById(id)).toList();
+
+  /// The current total price of all items.
+  int get totalPrice =>
+      items.fold(0, (total, current) => total + current.price);
+
+  /// Adds [item] to cart. This is the only way to modify the cart from outside.
+  void add(Item item) {
+    _itemIds.add(item.id);
+    // This line tells [Model] that it should rebuild the widgets that
+    // depend on it.
+    notifyListeners();
+  }
+}