add symbol check to defineProperty helper - fixes #1010

src/babel/transformation/templates/define-property.js

@@ -1,7 +1,7 @@
 (function (obj, key, value) {
   return Object.defineProperty(obj, key, {
     value: value,
-    enumerable: true,
+    enumerable: key == null || typeof Symbol == "undefined" || key.constructor !== Symbol,
     configurable: true,
     writable: true
   });