[Gallery] Fix directory structure (#312)

gallery/lib/demos/cupertino/cupertino_activity_indicator_demo.dart

@@ -0,0 +1,27 @@
+// 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 cupertinoActivityIndicatorDemo
+
+class CupertinoProgressIndicatorDemo extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return CupertinoPageScaffold(
+      navigationBar: CupertinoNavigationBar(
+        automaticallyImplyLeading: false,
+        middle: Text(
+          GalleryLocalizations.of(context).demoCupertinoActivityIndicatorTitle,
+        ),
+      ),
+      child: const Center(
+        child: CupertinoActivityIndicator(),
+      ),
+    );
+  }
+}
+
+// END