Don't unmark dictionary mode maps as unstable.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index fe791f308738b326c9d2dfd2ca56a0fa7fdf7a9d..246557bdfc044b05604cd19313b6025f1d8ea724 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -6682,7 +6682,9 @@ MaybeObject* Map::RawCopy(int instance_size) {
   new_bit_field3 = EnumLengthBits::update(new_bit_field3,
                                           kInvalidEnumCacheSentinel);
   new_bit_field3 = Deprecated::update(new_bit_field3, false);
-  new_bit_field3 = IsUnstable::update(new_bit_field3, false);
+  if (!is_dictionary_map()) {
+    new_bit_field3 = IsUnstable::update(new_bit_field3, false);
+  }
   result->set_bit_field3(new_bit_field3);
   return result;
 }