add descriptor falsy check to defaults helper - #739

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

@@ -3,7 +3,7 @@
   for (var i = 0; i < keys.length; i++) {
     var key = keys[i];
     var value = Object.getOwnPropertyDescriptor(defaults, key);
-    if (value.configurable && obj[key] === undefined) {
+    if (value && value.configurable && obj[key] === undefined) {
       Object.defineProperty(obj, key, value);
     }
   }