Merge pull request #739 from lukescott/defaults-fix

fix defaults helper in strict mode - #738

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

@@ -2,8 +2,9 @@
   var keys = Object.getOwnPropertyNames(defaults);
   for (var i = 0; i < keys.length; i++) {
     var key = keys[i];
-    if (obj[key] === undefined) {
-      Object.defineProperty(obj, key, Object.getOwnPropertyDescriptor(defaults, key));
+    var value = Object.getOwnPropertyDescriptor(defaults, key);
+    if (value.configurable && obj[key] === undefined) {
+      Object.defineProperty(obj, key, value);
     }
   }
   return obj;