[es6] Fix WeakMap/Set built-ins subclasssing.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 6da29f36d85660639499954ad09c7d58bf544b69..5c88e33d78736a6e53dc0c1027b691864895395d 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -17331,7 +17331,6 @@ void JSMap::Clear(Handle<JSMap> map) {
 
 void JSWeakCollection::Initialize(Handle<JSWeakCollection> weak_collection,
                                   Isolate* isolate) {
-  DCHECK_EQ(0, weak_collection->map()->GetInObjectProperties());
   Handle<ObjectHashTable> table = ObjectHashTable::New(isolate, 0);
   weak_collection->set_table(*table);
 }