Don't turn objects with empty-string properties into fast-mode.

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 15692 matching lines @@
     if (IsKey(k)) {
       Object* value = ValueAt(i);
       Name* key;
       if (k->IsSymbol()) {
         key = Symbol::cast(k);
       } else {
         // Ensure the key is a unique name before writing into the
         // instance descriptor.
         MaybeObject* maybe_key = heap->InternalizeString(String::cast(k));
         if (!maybe_key->To(&key)) return maybe_key;
+        if (key->Equals(heap->empty_string())) return this;
       }
 
       PropertyDetails details = DetailsAt(i);
       int enumeration_index = details.dictionary_index();
       PropertyType type = details.type();
 
       if (value->IsJSFunction()) {
         ConstantDescriptor d(key, value, details.attributes());
         descriptors->Set(enumeration_index - 1, &d, witness);
       } else if (type == NORMAL) {
@@ skipping 759 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