Implement a simple checkbox in Sky’s fn2 components library

sky/examples/widgets/main.sky

Index: sky/examples/widgets/main.sky
diff --git a/sky/examples/stocks/main.sky b/sky/examples/widgets/main.sky
similarity index 68%
copy from sky/examples/stocks/main.sky
copy to sky/examples/widgets/main.sky
index b57f262ef89605d4eb735965ad4e4174a3be883e..506b0ff837521d1945a72af0108950f539768c38 100644
--- a/sky/examples/stocks/main.sky
+++ b/sky/examples/widgets/main.sky
@@ -5,11 +5,11 @@
 // found in the LICENSE file.
 -->
 <sky>
+<import src="/sky/framework/debug/shake-to-reload.sky" />
 <script>
-// TODO(abarth): Should this be package:stocks/stocks_app.dart?
-import 'lib/stock_app.dart';
+import 'widgets_app.dart';
 void main() {
-  new StocksApp();
+  new WidgetsApp();
 }
 </script>
 </sky>