Update ObjectToString to Harmony-draft algorithm

src/weak-collection.js

Index: src/weak-collection.js
diff --git a/src/weak-collection.js b/src/weak-collection.js
index 1160176d66f6c34b8cf8acb59d0841d832055aae..273060adff58b6276d820d23376364e93759c185 100644
--- a/src/weak-collection.js
+++ b/src/weak-collection.js
@@ -114,6 +114,8 @@ function SetUpWeakMap() {
   %SetCode($WeakMap, WeakMapConstructor);
   %FunctionSetPrototype($WeakMap, new $Object());
   %AddNamedProperty($WeakMap.prototype, "constructor", $WeakMap, DONT_ENUM);
+  %AddNamedProperty(
+      $WeakMap.prototype, symbolToStringTag, "WeakMap", DONT_ENUM | READ_ONLY);
 
   // Set up the non-enumerable functions on the WeakMap prototype object.
   InstallFunctions($WeakMap.prototype, DONT_ENUM, $Array(
@@ -214,6 +216,8 @@ function SetUpWeakSet() {
   %SetCode($WeakSet, WeakSetConstructor);
   %FunctionSetPrototype($WeakSet, new $Object());
   %AddNamedProperty($WeakSet.prototype, "constructor", $WeakSet, DONT_ENUM);
+  %AddNamedProperty(
+      $WeakSet.prototype, symbolToStringTag, "WeakSet", DONT_ENUM | READ_ONLY);
 
   // Set up the non-enumerable functions on the WeakSet prototype object.
   InstallFunctions($WeakSet.prototype, DONT_ENUM, $Array(