Adding shrine. (#3)

Adds a new version of Shrine from the MDC codelabs.

shrine/lib/model/product.dart

@@ -0,0 +1,48 @@
+// 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/foundation.dart';
+
+enum Category {
+  all,
+  accessories,
+  clothing,
+  home,
+}
+
+class Product {
+  const Product({
+    @required this.category,
+    @required this.id,
+    @required this.isFeatured,
+    @required this.name,
+    @required this.price,
+  })  : assert(category != null),
+        assert(id != null),
+        assert(isFeatured != null),
+        assert(name != null),
+        assert(price != null);
+
+  final Category category;
+  final int id;
+  final bool isFeatured;
+  final String name;
+  final int price;
+
+  String get assetName => '$id-0.jpg';
+  String get assetPackage => 'shrine_images';
+
+  @override
+  String toString() => '$name (id=$id)';
+}