Testing sample (#500)

testing_app/lib/models/favorites.dart

@@ -0,0 +1,22 @@
+// Copyright 2020 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/material.dart';
+
+/// The [Favorites] class holds a list of favorite items saved by the user.
+class Favorites extends ChangeNotifier {
+  final List<int> _favoriteItems = [];
+
+  List<int> get items => _favoriteItems;
+
+  void add(int itemNo) {
+    _favoriteItems.add(itemNo);
+    notifyListeners();
+  }
+
+  void remove(int itemNo) {
+    _favoriteItems.remove(itemNo);
+    notifyListeners();
+  }
+}