fix defaults helper iteration - #738

lib/6to5/transformation/templates/defaults.js

@@ -1,6 +1,7 @@
 (function (obj, defaults) {
   var keys = Object.getOwnPropertyNames(defaults);
-  for (var key in keys) {
+  for (var i = 0; i < keys.length; i++) {
+    var key = keys[i];
     if (obj[key] === undefined) {
       Object.defineProperty(obj, key, Object.getOwnPropertyDescriptor(defaults, key));
     }