[Gallery] Fix directory structure (#312)

gallery/lib/demos/cupertino/cupertino_button_demo.dart

@@ -0,0 +1,44 @@
+// 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/cupertino.dart';
+
+import 'package:gallery/l10n/gallery_localizations.dart';
+
+// BEGIN cupertinoButtonDemo
+
+class CupertinoButtonDemo extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return CupertinoPageScaffold(
+      navigationBar: CupertinoNavigationBar(
+        automaticallyImplyLeading: false,
+        middle:
+            Text(GalleryLocalizations.of(context).demoCupertinoButtonsTitle),
+      ),
+      child: Center(
+        child: Column(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: [
+            CupertinoButton(
+              child: Text(
+                GalleryLocalizations.of(context).cupertinoButton,
+              ),
+              onPressed: () {},
+            ),
+            SizedBox(height: 16),
+            CupertinoButton.filled(
+              child: Text(
+                GalleryLocalizations.of(context).cupertinoButtonWithBackground,
+              ),
+              onPressed: () {},
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}
+
+// END