Implement well-known symbol `Symbol.toStringTag`

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.
 
 // This file relies on the fact that the following declarations have been made
 // in runtime.js:
 // var $Object = global.Object;
 // var $Boolean = global.Boolean;
 // var $Number = global.Number;
 // var $Function = global.Function;
@@ skipping 197 matching lines @@
     "eval", GlobalEval
   ));
 }
 
 SetUpGlobal();
 
 
 // ----------------------------------------------------------------------------
 // Object
 
-// ECMA-262 - 15.2.4.2
+// ES6 draft Rev 26 - 19.1.3.6
+var reservedTags = ["Arguments", "Array", "Boolean", "Date",
+                    "Error", "Function", "Number", "RegExp", "String"];
 function ObjectToString() {
   if (IS_UNDEFINED(this) && !IS_UNDETECTABLE(this)) return "[object Undefined]";
   if (IS_NULL(this)) return "[object Null]";
-  return "[object " + %_ClassOf(ToObject(this)) + "]";
+  var O = ToObject(this);
+  var builtinTag = %_ClassOf(O);
+  if(reservedTags.indexOf(builtinTag) === -1){

[arv (Not doing code reviews), 2014/08/11 14:40:40]

Can you change this to a switch? We do not want a linear search for all of
these.

+    builtinTag = "Object";
+  }
+  var tag = builtinTag;
+  var hasTag = %HasProperty(O, symbolToStringTag);
+  if(hasTag){
+    tag = O[symbolToStringTag];
+    if(!IS_STRING(tag)){
+      tag = "???";
+    }
+    if(tag !== builtinTag && reservedTags.indexOf(tag) > -1){
+      tag = "~" + tag;
+    }
+  }
+  return "[object " + tag + "]";
 }
 
 
 // ECMA-262 - 15.2.4.3
 function ObjectToLocaleString() {
   CHECK_OBJECT_COERCIBLE(this, "Object.prototype.toLocaleString");
   return this.toString();
 }
 
 
@@ skipping 1635 matching lines @@
   %SetCode($Function, FunctionConstructor);
   %AddNamedProperty($Function.prototype, "constructor", $Function, DONT_ENUM);
 
   InstallFunctions($Function.prototype, DONT_ENUM, $Array(
     "bind", FunctionBind,
     "toString", FunctionToString
   ));
 }
 
 SetUpFunction();