Give correct RegExp flags in toString

When the flags property is not available on RegExp, the internals
regexp-flags implementation should be used regardless of DESCRIPTORS.

Fixes: zloirock#536

packages/core-js/modules/es.regexp.to-string.js

@@ -2,7 +2,6 @@
 var anObject = require('../internals/an-object');
 var fails = require('../internals/fails');
 var flags = require('../internals/regexp-flags');
-var DESCRIPTORS = require('../internals/descriptors');
 var TO_STRING = 'toString';
 var nativeToString = /./[TO_STRING];
 
@@ -16,6 +15,6 @@ if (NOT_GENERIC || INCORRECT_NAME) {
   require('../internals/redefine')(RegExp.prototype, TO_STRING, function toString() {
     var R = anObject(this);
     return '/'.concat(R.source, '/',
-      'flags' in R ? R.flags : !DESCRIPTORS && R instanceof RegExp ? flags.call(R) : undefined);
+      'flags' in R || !(R instanceof RegExp) ? R.flags : flags.call(R));
   }, { unsafe: true });
 }