Refactor fn2.dart, since it breached our 1000-line threshold.

sky/sdk/lib/framework/widgets/checkbox.dart

Index: sky/sdk/lib/framework/widgets/checkbox.dart
diff --git a/sky/sdk/lib/framework/components2/checkbox.dart b/sky/sdk/lib/framework/widgets/checkbox.dart
similarity index 97%
rename from sky/sdk/lib/framework/components2/checkbox.dart
rename to sky/sdk/lib/framework/widgets/checkbox.dart
index 36fd278dbdbe9bfe133dc2d80b550616bc1a35c7..1d2630c48487e9f77f3d34126202361c8a9c698e 100644
--- a/sky/sdk/lib/framework/components2/checkbox.dart
+++ b/sky/sdk/lib/framework/widgets/checkbox.dart
@@ -2,13 +2,13 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:sky' as sky;
+
 import 'package:sky/framework/theme2/colors.dart' as colors;
 
-import 'dart:sky' as sky;
-import '../fn2.dart';
 import '../rendering/box.dart';
-import '../rendering/object.dart';
 import 'button_base.dart';
+import 'wrappers.dart';
 
 typedef void ValueChanged(value);