[Gallery] Fix directory structure (#312)

gallery/lib/studies/crane/backlayer.dart

@@ -0,0 +1,49 @@
+// 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';
+
+abstract class BackLayerItem extends StatefulWidget {
+  final int index;
+
+  BackLayerItem({Key key, this.index}) : super(key: key);
+}
+
+class BackLayer extends StatefulWidget {
+  final List<BackLayerItem> backLayerItems;
+  final TabController tabController;
+
+  const BackLayer({Key key, this.backLayerItems, this.tabController})
+      : super(key: key);
+
+  @override
+  _BackLayerState createState() => _BackLayerState();
+}
+
+class _BackLayerState extends State<BackLayer> {
+  @override
+  void initState() {
+    super.initState();
+    widget.tabController.addListener(() => setState(() {}));
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final tabIndex = widget.tabController.index;
+    return DefaultFocusTraversal(
+      policy: WidgetOrderFocusTraversalPolicy(),
+      child: IndexedStack(
+        index: tabIndex,
+        children: [
+          for (BackLayerItem backLayerItem in widget.backLayerItems)
+            Focus(
+              canRequestFocus: backLayerItem.index == tabIndex,
+              debugLabel: 'backLayerItem: $backLayerItem',
+              child: backLayerItem,
+            )
+        ],
+      ),
+    );
+  }
+}