Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/ia32/macro-assembler-ia32.cc

 // 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 1803 matching lines @@
   cmp(object, Operand::StaticVariable(new_space_allocation_top));
   Check(below, kUndoAllocationOfNonAllocatedMemory);
 #endif
   mov(Operand::StaticVariable(new_space_allocation_top), object);
 }
 
 
 void MacroAssembler::AllocateHeapNumber(Register result,
                                         Register scratch1,
                                         Register scratch2,
-                                        Label* gc_required) {
+                                        Label* gc_required,
+                                        MutableMode mode) {
   // Allocate heap number in new space.
   Allocate(HeapNumber::kSize, result, scratch1, scratch2, gc_required,
            TAG_OBJECT);
 
+  Handle<Map> map = mode == MUTABLE
+      ? isolate()->factory()->mutable_heap_number_map()
+      : isolate()->factory()->heap_number_map();
+
   // Set the map.
-  mov(FieldOperand(result, HeapObject::kMapOffset),
-      Immediate(isolate()->factory()->heap_number_map()));
+  mov(FieldOperand(result, HeapObject::kMapOffset), Immediate(map));
 }
 
 
 void MacroAssembler::AllocateTwoByteString(Register result,
                                            Register length,
                                            Register scratch1,
                                            Register scratch2,
                                            Register scratch3,
                                            Label* gc_required) {
   // Calculate the number of bytes needed for the characters in the string while
@@ skipping 1804 matching lines @@
   if (ms.shift() > 0) sar(edx, ms.shift());
   mov(eax, dividend);
   shr(eax, 31);
   add(edx, eax);
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32