Speed up native error check.

src/api.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2291 matching lines @@
   i::Handle<i::String> symbol =
       isolate->factory()->LookupAsciiSymbol(builtin_name);
   i::Handle<i::JSBuiltinsObject> builtins = isolate->js_builtins_object();
   return builtins->GetPropertyNoExceptionThrown(*symbol);
 }
 
 
 static bool CheckConstructor(i::Isolate* isolate,
                              i::Handle<i::JSObject> obj,
                              const char* class_name) {
-  return obj->map()->constructor() == LookupBuiltin(isolate, class_name);
+  i::Object* constr = obj->map()->constructor();
+  if (!constr->IsJSFunction()) return false;
+  i::JSFunction* func = i::JSFunction::cast(constr);
+  return func->shared()->native() &&
+         constr == LookupBuiltin(isolate, class_name);
 }
 
 
 bool Value::IsNativeError() const {
   i::Isolate* isolate = i::Isolate::Current();
   if (IsDeadCheck(isolate, "v8::Value::IsNativeError()")) return false;
   i::Handle<i::Object> obj = Utils::OpenHandle(this);
   if (obj->IsJSObject()) {
     i::Handle<i::JSObject> js_obj(i::JSObject::cast(*obj));
     return CheckConstructor(isolate, js_obj, "$Error") ||
@@ skipping 4355 matching lines @@
 
   v->VisitPointers(blocks_.first(), first_block_limit_);
 
   for (int i = 1; i < blocks_.length(); i++) {
     v->VisitPointers(blocks_[i], &blocks_[i][kHandleBlockSize]);
   }
 }
 
 
 } }  // namespace v8::internal