Move 'use strict' to file level

Change-Id: I1f0dae4aa168e77c112bcd3688c98b84e4ce5f36
diff --git a/dev/js/src/buttongroup.js b/dev/js/src/buttongroup.js
index 7c8d8c6..0ea1d38 100644
--- a/dev/js/src/buttongroup.js
+++ b/dev/js/src/buttongroup.js
@@ -1,5 +1,6 @@
+"use strict";
+
 define(['buttongroup/menu','menu/item','util'], function (treeMenuClass, defaultItemClass) {
-  "use strict";
   
   return {
     /**