Do not use js builtins object to determine whether a function is a builtin.

src/v8natives.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 (function(global, utils) {
 
 %CheckIsBootstrapping();
 
 // ----------------------------------------------------------------------------
 // Imports
@@ skipping 1604 matching lines @@
 
 function FunctionSourceString(func) {
   while (%IsJSFunctionProxy(func)) {
     func = %GetCallTrap(func);
   }
 
   if (!IS_FUNCTION(func)) {
     throw MakeTypeError(kNotGeneric, 'Function.prototype.toString');
   }
 
-  if (%FunctionIsBuiltin(func)) {
+  if (%FunctionHidesSource(func)) {
     return NativeCodeFunctionSourceString(func);
   }
 
   var classSource = %ClassGetSourceCode(func);
   if (IS_STRING(classSource)) {
     return classSource;
   }
 
   var source = %FunctionGetSourceCode(func);
   if (!IS_STRING(source)) {
@@ skipping 174 matching lines @@
 });
 
 utils.ExportToRuntime(function(to) {
   to.GlobalEval = GlobalEval;
   to.ObjectDefineOwnProperty = DefineOwnPropertyFromAPI;
   to.ObjectGetOwnPropertyDescriptor = ObjectGetOwnPropertyDescriptor;
   to.ToCompletePropertyDescriptor = ToCompletePropertyDescriptor;
 });
 
 })