Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

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 412 matching lines @@
     if (object.is_null()) return EXCEPTION;
   }
 
   if (object->IsSmi()) {
     if (deferred_string_key) SerializeDeferredKey(comma, key);
     return SerializeSmi(Smi::cast(*object));
   }
 
   switch (HeapObject::cast(*object)->map()->instance_type()) {
     case HEAP_NUMBER_TYPE:
+    case MUTABLE_HEAP_NUMBER_TYPE:
       if (deferred_string_key) SerializeDeferredKey(comma, key);
       return SerializeHeapNumber(Handle<HeapNumber>::cast(object));
     case ODDBALL_TYPE:
       switch (Oddball::cast(*object)->kind()) {
         case Oddball::kFalse:
           if (deferred_string_key) SerializeDeferredKey(comma, key);
           AppendAscii("false");
           return SUCCESS;
         case Oddball::kTrue:
           if (deferred_string_key) SerializeDeferredKey(comma, key);
@@ skipping 434 matching lines @@
       SerializeString_<false, uint8_t>(object);
     } else {
       SerializeString_<false, uc16>(object);
     }
   }
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_JSON_STRINGIFIER_H_