Fix errors in Sky detected by Dart analyzer

sky/framework/components/checkbox.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import '../fn.dart';
 import 'button_base.dart';
 
 typedef void ValueChanged(value);
 
 class Checkbox extends ButtonBase {
@@ skipping 61 matching lines @@
       children: [
         new Container(
           styles: [highlight ? _containerHighlightStyle : _containerStyle],
           children: [
             new Container(
               styles: [checked ? _checkedStyle : _uncheckedStyle]
             )
           ]
         )
       ]
-    )
+    );
   }
 }