Update ObjectToString to Harmony-draft algorithm

src/symbol.js

Index: src/symbol.js
diff --git a/src/symbol.js b/src/symbol.js
index ce3327bbdbaf7beac83a07073c49ac64f869a863..e3b603bceeb87645d85a5a7f5b34d1cae54770a4 100644
--- a/src/symbol.js
+++ b/src/symbol.js
@@ -104,7 +104,7 @@ function SetUpSymbol() {
     // "isConcatSpreadable", symbolIsConcatSpreadable,
     // "isRegExp", symbolIsRegExp,
     "iterator", symbolIterator,
-    // "toStringTag", symbolToStringTag,
+    "toStringTag", symbolToStringTag,
     "unscopables", symbolUnscopables
   ));
   InstallFunctions($Symbol, DONT_ENUM, $Array(
@@ -113,6 +113,8 @@ function SetUpSymbol() {
   ));
 
   %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