Add a Material/Cupertino adaptive application example (#69)

platform_design/test/widget_test.dart

@@ -0,0 +1,30 @@
+import 'package:flutter/foundation.dart';
+import 'package:flutter/cupertino.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:platform_design/main.dart';
+
+void main() {
+  testWidgets('Can change platform correctly', (tester) async {
+    await tester.pumpWidget(MyAdaptingApp());
+
+    // The test should be able to find the drawer button.
+    expect(find.byIcon(Icons.menu), findsOneWidget);
+    // There should be a refresh button.
+    expect(find.byIcon(Icons.refresh), findsOneWidget);
+
+    debugDefaultTargetPlatformOverride = TargetPlatform.iOS;
+    await tester.pumpWidget(MyAdaptingApp());
+
+    // There should now be a large title style nav bar.
+    expect(find.byType(CupertinoSliverNavigationBar), findsOneWidget);
+    // There's a tab button for the first tab.
+    expect(find.byIcon(CupertinoIcons.music_note), findsOneWidget);
+    // The hamburger button isn't there anymore.
+    expect(find.byIcon(Icons.menu), findsNothing);
+
+    // Since this is a static, undo the change made in the test.
+    debugDefaultTargetPlatformOverride = null;
+  });
+}