[es6] don't "replace" Object.prototype.toString for --harmony-tostring

src/messages.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.
 
 // -------------------------------------------------------------------
 
 var kMessages = {
   // Error
   cyclic_proto:                  ["Cyclic __proto__ value"],
   code_gen_from_strings:         ["%0"],
@@ skipping 230 matching lines @@
   if (IS_FUNCTION(obj)) {
     var str = %_CallFunction(obj, FunctionToString);
     if (str.length > 128) {
       str = %_SubString(str, 0, 111) + "...<omitted>..." +
             %_SubString(str, str.length - 2, str.length);
     }
     return str;
   }
   if (IS_SYMBOL(obj)) return %_CallFunction(obj, $symbolToString);
   if (IS_OBJECT(obj)
-      && %GetDataProperty(obj, "toString") === DefaultObjectToString) {
+      && %GetDataProperty(obj, "toString") === ObjectToString) {
     var constructor = %GetDataProperty(obj, "constructor");
     if (typeof constructor == "function") {
       var constructorName = constructor.name;
       if (IS_STRING(constructorName) && constructorName !== "") {
         return "#<" + constructorName + ">";
       }
     }
   }
   if (CanBeSafelyTreatedAsAnErrorObject(obj)) {
     return %_CallFunction(obj, ErrorToString);
@@ skipping 1019 matching lines @@
 function SetUpStackOverflowBoilerplate() {
   var boilerplate = MakeRangeError('stack_overflow', []);
 
   %DefineAccessorPropertyUnchecked(
       boilerplate, 'stack', StackTraceGetter, StackTraceSetter, DONT_ENUM);
 
   return boilerplate;
 }
 
 var kStackOverflowBoilerplate = SetUpStackOverflowBoilerplate();