Use mutable heapnumbers to store doubles in fields.

src/json-stringifier.h

 // 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 626 matching lines @@
     for (int i = 0; i < map->NumberOfOwnDescriptors(); i++) {
       Handle<Name> name(map->instance_descriptors()->GetKey(i), isolate_);
       // TODO(rossberg): Should this throw?
       if (!name->IsString()) continue;
       Handle<String> key = Handle<String>::cast(name);
       PropertyDetails details = map->instance_descriptors()->GetDetails(i);
       if (details.IsDontEnum() || details.IsDeleted()) continue;
       Handle<Object> property;
       if (details.type() == FIELD && *map == object->map()) {
         property = Handle<Object>(
-                       object->FastPropertyAt(
+                       object->RawFastPropertyAt(
                            map->instance_descriptors()->GetFieldIndex(i)),
                        isolate_);
       } else {
         property = GetProperty(isolate_, object, key);
         if (property.is_null()) return EXCEPTION;
       }
       Result result = SerializeProperty(property, comma, key);
       if (!comma && result == SUCCESS) comma = true;
       if (result >= EXCEPTION) return result;
     }
@@ skipping 181 matching lines @@
       SerializeString_<false, uint8_t>(object);
     } else {
       SerializeString_<false, uc16>(object);
     }
   }
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_JSON_STRINGIFIER_H_