Update ObjectToString to Harmony-draft algorithm

src/symbol.js

 // Copyright 2013 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.
 
 "use strict";
 
 // This file relies on the fact that the following declaration has been made
 // in runtime.js:
 // var $Array = global.Array;
 
@@ skipping 86 matching lines @@
 
   %SetCode($Symbol, SymbolConstructor);
   %FunctionSetPrototype($Symbol, new $Object());
 
   InstallConstants($Symbol, $Array(
     // TODO(rossberg): expose when implemented.
     // "hasInstance", symbolHasInstance,
     // "isConcatSpreadable", symbolIsConcatSpreadable,
     // "isRegExp", symbolIsRegExp,
     "iterator", symbolIterator,
-    // "toStringTag", symbolToStringTag,
+    "toStringTag", symbolToStringTag,
     "unscopables", symbolUnscopables
   ));
   InstallFunctions($Symbol, DONT_ENUM, $Array(
     "for", SymbolFor,
     "keyFor", SymbolKeyFor
   ));
 
   %AddNamedProperty($Symbol.prototype, "constructor", $Symbol, DONT_ENUM);
+  %AddNamedProperty(
+      $Symbol.prototype, symbolToStringTag, "Symbol", DONT_ENUM | READ_ONLY);
   InstallFunctions($Symbol.prototype, DONT_ENUM, $Array(
     "toString", SymbolToString,
     "valueOf", SymbolValueOf
   ));
 }
 
 SetUpSymbol();
 
 
 function ExtendObject() {
   %CheckIsBootstrapping();
 
   InstallFunctions($Object, DONT_ENUM, $Array(
     "getOwnPropertySymbols", ObjectGetOwnPropertySymbols
   ));
 }
 
 ExtendObject();