Use an invalid identifier as the es6-shim iterator key so it doesn’t …

…show up in the console as easily.

es6-shim.js

@@ -122,8 +122,7 @@
   // work properly with each other, even though we don't have full Iterator
   // support.  That is, `Array.from(map.keys())` will work, but we don't
   // pretend to export a "real" Iterator interface.
-  var $iterator$ = (typeof Symbol === 'function' && Symbol.iterator) ||
-    '_es6shim_iterator_';
+  var $iterator$ = (typeof Symbol === 'function' && Symbol.iterator) || '_es6-shim iterator_';
   // Firefox ships a partial implementation using the name @@iterator.
   // https://bugzilla.mozilla.org/show_bug.cgi?id=907077#c14
   // So use that name if we detect it.