Use mutable heapnumbers to store doubles in fields.

src/bootstrapper.cc

 // Copyright 2012 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 2398 matching lines @@
   if (from->HasFastProperties()) {
     Handle<DescriptorArray> descs =
         Handle<DescriptorArray>(from->map()->instance_descriptors());
     for (int i = 0; i < descs->number_of_descriptors(); i++) {
       PropertyDetails details = descs->GetDetails(i);
       switch (details.type()) {
         case FIELD: {
           HandleScope inner(isolate());
           Handle<Name> key = Handle<Name>(descs->GetKey(i));
           int index = descs->GetFieldIndex(i);
-          Handle<Object> value = Handle<Object>(from->FastPropertyAt(index),
+          ASSERT(!descs->GetDetails(i).representation().IsDouble());
+          Handle<Object> value = Handle<Object>(from->RawFastPropertyAt(index),
                                                 isolate());
           CHECK_NOT_EMPTY_HANDLE(isolate(),
                                  JSObject::SetLocalPropertyIgnoreAttributes(
                                      to, key, value, details.attributes()));
           break;
         }
         case CONSTANT_FUNCTION: {
           HandleScope inner(isolate());
           Handle<Name> key = Handle<Name>(descs->GetKey(i));
           Handle<JSFunction> fun =
@@ skipping 189 matching lines @@
   return from + sizeof(NestingCounterType);
 }
 
 
 // Called when the top-level V8 mutex is destroyed.
 void Bootstrapper::FreeThreadResources() {
   ASSERT(!IsActive());
 }
 
 } }  // namespace v8::internal