Use mutable heapnumbers to store doubles in fields.

src/factory.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 249 matching lines @@
 
   // Numbers (e.g. literals) are pretenured by the parser.
   Handle<Object> NewNumber(double value,
                            PretenureFlag pretenure = NOT_TENURED);
 
   Handle<Object> NewNumberFromInt(int32_t value,
                                   PretenureFlag pretenure = NOT_TENURED);
   Handle<Object> NewNumberFromUint(uint32_t value,
                                   PretenureFlag pretenure = NOT_TENURED);
 
+  Handle<HeapNumber> NewHeapNumber(double value,
+                                   PretenureFlag pretenure = NOT_TENURED);
+
   // These objects are used by the api to create env-independent data
   // structures in the heap.
   Handle<JSObject> NewNeanderObject();
 
   Handle<JSObject> NewArgumentsObject(Handle<Object> callee, int length);
 
   // JS objects are pretenured when allocated by the bootstrapper and
   // runtime.
   Handle<JSObject> NewJSObject(Handle<JSFunction> constructor,
                                PretenureFlag pretenure = NOT_TENURED);
@@ skipping 252 matching lines @@
   // Update the map cache in the native context with (keys, map)
   Handle<MapCache> AddToMapCache(Handle<Context> context,
                                  Handle<FixedArray> keys,
                                  Handle<Map> map);
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_FACTORY_H_