[Gallery] Fix directory structure (#312)

gallery/lib/layout/focus_traversal_policy.dart

@@ -0,0 +1,59 @@
+// 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';
+
+/// [EdgeChildrenFocusTraversalPolicy] can be used to make sure that when you
+/// are at the last or first focus node inside of a focus scope, we'll request
+/// focus for given focus node outside of the scope.
+///
+/// This can be used to for example make sure that you can request focus outside
+/// of the MaterialApp you are currently in.
+class EdgeChildrenFocusTraversalPolicy extends WidgetOrderFocusTraversalPolicy {
+  EdgeChildrenFocusTraversalPolicy({
+    @required this.firstFocusNodeOutsideScope,
+    @required this.lastFocusNodeOutsideScope,
+    this.focusScope,
+    this.firstFocusNodeInsideScope,
+    this.lastFocusNodeInsideScope,
+  }) : assert((focusScope != null &&
+                firstFocusNodeInsideScope == null &&
+                lastFocusNodeInsideScope == null) ||
+            (firstFocusNodeInsideScope != null &&
+                lastFocusNodeInsideScope != null &&
+                focusScope == null));
+
+  final FocusNode firstFocusNodeOutsideScope;
+  final FocusNode lastFocusNodeOutsideScope;
+
+  /// Either provide [focusScope] or both [firstFocusNodeInsideScope]
+  /// and [lastFocusNodeInsideScope].
+  final FocusScopeNode focusScope;
+  final FocusNode firstFocusNodeInsideScope;
+  final FocusNode lastFocusNodeInsideScope;
+
+  @override
+  bool previous(FocusNode currentNode) {
+    if (currentNode ==
+        (firstFocusNodeInsideScope ??
+            focusScope.traversalChildren.toList().first)) {
+      firstFocusNodeOutsideScope.requestFocus();
+      return true;
+    } else {
+      return super.previous(currentNode);
+    }
+  }
+
+  @override
+  bool next(FocusNode currentNode) {
+    if (currentNode ==
+        (lastFocusNodeInsideScope ??
+            focusScope.traversalChildren.toList().last)) {
+      lastFocusNodeOutsideScope.requestFocus();
+      return true;
+    } else {
+      return super.next(currentNode);
+    }
+  }
+}