fix buggy iterators detection

Author: zloirock

packages/core-js/internals/define-iterator.js

@@ -9,7 +9,7 @@ var createIteratorConstructor = require('../internals/create-iterator-constructo
 var setToStringTag = require('../internals/set-to-string-tag');
 var getPrototypeOf = require('../internals/object-get-prototype-of');
 var ITERATOR = require('../internals/well-known-symbol')('iterator');
-var BUGGY = !([].keys && 'next' in [].keys()); // Safari has buggy iterators w/o `next`
+var BUGGY = !![].keys && !('next' in [].keys()); // Safari has buggy iterators w/o `next`
 var FF_ITERATOR = '@@iterator';
 var KEYS = 'keys';
 var VALUES = 'values';