Implement well-known symbol `Symbol.toStringTag`

src/v8natives.js

Index: src/v8natives.js
diff --git a/src/v8natives.js b/src/v8natives.js
index 8484150248a13a2ef01467da31603fab1db88b0c..9dad17643ddc840c289ebf18f92d623aa7fdf60b 100644
--- a/src/v8natives.js
+++ b/src/v8natives.js
@@ -215,11 +215,29 @@ 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 + "]";
 }