So siehts ganz gut aus !

node_modules/core-js/modules/_keyof.js

@@ -0,0 +1,10 @@
+var getKeys = require('./_object-keys');
+var toIObject = require('./_to-iobject');
+module.exports = function (object, el) {
+  var O = toIObject(object);
+  var keys = getKeys(O);
+  var length = keys.length;
+  var index = 0;
+  var key;
+  while (length > index) if (O[key = keys[index++]] === el) return key;
+};