Implement well-known symbol `Symbol.toStringTag`

src/symbol.js

Index: src/symbol.js
diff --git a/src/symbol.js b/src/symbol.js
index 9f2e563704a0e678f7001b839213941e050b13ea..78216767e1427e95ae3a3d1aed8f17783fea291e 100644
--- a/src/symbol.js
+++ b/src/symbol.js
@@ -103,8 +103,8 @@ function SetUpSymbol() {
     // "hasInstance", symbolHasInstance,
     // "isConcatSpreadable", symbolIsConcatSpreadable,
     // "isRegExp", symbolIsRegExp,
-    "iterator", symbolIterator
-    // "toStringTag", symbolToStringTag,
+    "iterator", symbolIterator,
+    "toStringTag", symbolToStringTag
     // "unscopables", symbolUnscopables  // added in unscopables.js
   ));
   InstallFunctions($Symbol, DONT_ENUM, $Array(
@@ -121,6 +121,22 @@ function SetUpSymbol() {
 
 SetUpSymbol();
 
+function addToStringTagProperty(obj, tag){
+  %AddNamedProperty(obj, symbolToStringTag, tag, DONT_ENUM | READ_ONLY);
+}
+
+function setUpToStringTags(){
+  addToStringTagProperty(global.Math, "Math");
+  addToStringTagProperty(global.JSON, "JSON");
+  addToStringTagProperty($Symbol.prototype, "Symbol");
+  addToStringTagProperty(global.Promise.prototype, "Promise");
+  addToStringTagProperty(global.WeakMap.prototype, "WeakMap");
+  addToStringTagProperty(global.WeakSet.prototype, "WeakSet");
+  addToStringTagProperty(global.ArrayBuffer.prototype, "ArrayBuffer");
+  addToStringTagProperty(global.DataView.prototype, "DataView");
+}
+
+setUpToStringTags();
 
 function ExtendObject() {
   %CheckIsBootstrapping();