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/test/widget_test.dart

@@ -0,0 +1,29 @@
+// 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/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:provider_shopper/main.dart';
+
+void main() {
+  testWidgets('smoke test', (tester) async {
+    // Build our app and trigger a frame.
+    await tester.pumpWidget(MyApp());
+
+    // Check that shopping cart is empty at start.
+    await tester.tap(find.byIcon(Icons.shopping_cart));
+    await tester.pumpAndSettle();
+    expect(find.text('\$0'), findsOneWidget);
+
+    // Buy an item.
+    await tester.pageBack();
+    await tester.pumpAndSettle();
+    await tester.tap(find.text('ADD').first);
+
+    // Check that the shopping cart is not empty anymore.
+    await tester.tap(find.byIcon(Icons.shopping_cart));
+    await tester.pumpAndSettle();
+    expect(find.text('\$0'), findsNothing);
+  });
+}