Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/factory.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 1009 matching lines @@
 
 
 Handle<HeapNumber> Factory::NewHeapNumber(double value,
                                           PretenureFlag pretenure) {
   CALL_HEAP_FUNCTION(
       isolate(),
       isolate()->heap()->AllocateHeapNumber(value, pretenure), HeapNumber);
 }
 
 
+Handle<HeapNumber> Factory::NewMutableHeapNumber(double value,
+                                                 PretenureFlag pretenure) {
+  CALL_HEAP_FUNCTION(
+      isolate(),
+      isolate()->heap()->AllocateMutableHeapNumber(value, pretenure),
+      HeapNumber);
+}
+
+
 Handle<JSObject> Factory::NewNeanderObject() {
   CALL_HEAP_FUNCTION(
       isolate(),
       isolate()->heap()->AllocateJSObjectFromMap(
           isolate()->heap()->neander_map()),
       JSObject);
 }
 
 
 Handle<Object> Factory::NewTypeError(const char* message,
@@ skipping 977 matching lines @@
   return Handle<Object>::null();
 }
 
 
 Handle<Object> Factory::ToBoolean(bool value) {
   return value ? true_value() : false_value();
 }
 
 
 } }  // namespace v8::internal