Use mutable heapnumbers to store doubles in fields.

src/heap-snapshot-generator.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 1314 matching lines @@
                   NULL,
                   js_obj->GetInObjectPropertyOffset(index));
             } else {
               TagObject(value, "(hidden properties)");
               SetInternalReference(
                   js_obj, entry,
                   "hidden_properties", value,
                   js_obj->GetInObjectPropertyOffset(index));
             }
           } else {
-            Object* value = js_obj->FastPropertyAt(index);
+            Object* value = js_obj->RawFastPropertyAt(index);
             if (k != heap_->hidden_string()) {
               SetPropertyReference(js_obj, entry, k, value);
             } else {
               TagObject(value, "(hidden properties)");
               SetInternalReference(js_obj, entry, "hidden_properties", value);
             }
           }
           break;
         }
         case CONSTANT_FUNCTION:
@@ skipping 1313 matching lines @@
 
 
 void HeapSnapshotJSONSerializer::SortHashMap(
     HashMap* map, List<HashMap::Entry*>* sorted_entries) {
   for (HashMap::Entry* p = map->Start(); p != NULL; p = map->Next(p))
     sorted_entries->Add(p);
   sorted_entries->Sort(SortUsingEntryValue);
 }
 
 } }  // namespace v8::internal