Don't unmark dictionary mode maps as unstable.

src/objects.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 6664 matching lines @@
   result->set_prototype(prototype());
   result->set_constructor(constructor());
   result->set_bit_field(bit_field());
   result->set_bit_field2(bit_field2());
   int new_bit_field3 = bit_field3();
   new_bit_field3 = OwnsDescriptors::update(new_bit_field3, true);
   new_bit_field3 = NumberOfOwnDescriptorsBits::update(new_bit_field3, 0);
   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;
 }
 
 
 Handle<Map> Map::CopyNormalized(Handle<Map> map,
                                 PropertyNormalizationMode mode,
                                 NormalizedMapSharingMode sharing) {
   int new_instance_size = map->instance_size();
   if (mode == CLEAR_INOBJECT_PROPERTIES) {
@@ skipping 9780 matching lines @@
 #define ERROR_MESSAGES_TEXTS(C, T) T,
   static const char* error_messages_[] = {
       ERROR_MESSAGES_LIST(ERROR_MESSAGES_TEXTS)
   };
 #undef ERROR_MESSAGES_TEXTS
   return error_messages_[reason];
 }
 
 
 } }  // namespace v8::internal