Fix ES6-ism in addon

addon/display/panel.js

@@ -114,7 +114,7 @@
   }
 
   function isAtTop(cm, dom) {
-    for (let sibling = dom.nextSibling; sibling; sibling = sibling.nextSibling)
+    for (var sibling = dom.nextSibling; sibling; sibling = sibling.nextSibling)
       if (sibling == cm.getWrapperElement()) return true
     return false
   }