better symbol iterator check in regenerator runtime

lib/6to5/transformation/transformers/es6-generators/runtime.js

@@ -266,13 +266,9 @@ runtime.keys = function (object) {
   };
 };
 
-function isIn(key, obj) {
-  return typeof obj !== "string" && typeof obj !== "number" && key in obj;
-}
-
 function values(iterable) {
   var iterator = iterable;
-  if (iteratorSymbol in iterable) {
+  if (iterable[iteratorSymbol]) {
     iterator = iterable[iteratorSymbol]();
   } else if (!isNaN(iterable.length)) {
     var i = -1;