Implement well-known symbol `Symbol.toStringTag`

src/collection.js

Index: src/collection.js
diff --git a/src/collection.js b/src/collection.js
index e5fc0b33c8813a8071b85871d59166eca6d76eee..f2a8546eee63cf14966b716a869ced8005ebc729 100644
--- a/src/collection.js
+++ b/src/collection.js
@@ -148,6 +148,7 @@ function SetUpSet() {
   %SetCode($Set, SetConstructor);
   %FunctionSetPrototype($Set, new $Object());
   %AddNamedProperty($Set.prototype, "constructor", $Set, DONT_ENUM);
+  %AddNamedProperty($Set.prototype, symbolToStringTag, "Set",  DONT_ENUM | READ_ONLY);
 
   %FunctionSetLength(SetForEach, 1);
 
@@ -283,6 +284,7 @@ function SetUpMap() {
   %SetCode($Map, MapConstructor);
   %FunctionSetPrototype($Map, new $Object());
   %AddNamedProperty($Map.prototype, "constructor", $Map, DONT_ENUM);
+  %AddNamedProperty($Map.prototype, symbolToStringTag, "Map",  DONT_ENUM | READ_ONLY);
 
   %FunctionSetLength(MapForEach, 1);