restructured the add to app samples (#698)

add_to_app/plugin/flutter_module_using_plugin/test/widget_test.dart

@@ -0,0 +1,49 @@
+// This is a basic Flutter widget test.
+//
+// To perform an interaction with a widget in your test, use the WidgetTester
+// utility that Flutter provides. For example, you can send tap and scroll
+// gestures. You can also use WidgetTester to find child widgets in the widget
+// tree, read text, and verify that the values of widget properties are correct.
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:flutter_module_using_plugin/main.dart';
+import 'package:provider/provider.dart';
+
+class MockCounterModel extends ChangeNotifier implements CounterModel {
+  int _count = 0;
+
+  int get count => _count;
+
+  void increment() {
+    _count++;
+    notifyListeners();
+  }
+}
+
+void main() {
+  testWidgets('MiniView smoke test', (WidgetTester tester) async {
+    // Build our app and trigger a frame.
+    await tester.pumpWidget(
+      MaterialApp(
+        home: ChangeNotifierProvider<CounterModel>.value(
+          value: MockCounterModel(),
+          child: Contents(),
+        ),
+      ),
+    );
+
+    // Verify that our counter starts at 0.
+    expect(find.text('Taps: 0'), findsOneWidget);
+    expect(find.text('Taps: 1'), findsNothing);
+
+    // Tap the '+' icon and trigger a frame.
+    await tester.tap(find.text('Tap me!'));
+    await tester.pump();
+
+    // Verify that our counter has incremented.
+    expect(find.text('Taps: 0'), findsNothing);
+    expect(find.text('Taps: 1'), findsOneWidget);
+  });
+}