Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/ia32/macro-assembler-ia32.cc

Index: src/ia32/macro-assembler-ia32.cc
diff --git a/src/ia32/macro-assembler-ia32.cc b/src/ia32/macro-assembler-ia32.cc
index 0235a13894bd08bcf84d9d9f8f06cf4009ed26a2..9ac9e9e2bd0384a8debc8a2ac03111f39049c908 100644
--- a/src/ia32/macro-assembler-ia32.cc
+++ b/src/ia32/macro-assembler-ia32.cc
@@ -1832,6 +1832,20 @@ void MacroAssembler::AllocateHeapNumber(Register result,
 }
 
 
+void MacroAssembler::AllocateMutableHeapNumber(Register result,
+                                               Register scratch1,
+                                               Register scratch2,
+                                               Label* gc_required) {
+  // Allocate heap number in new space.
+  Allocate(HeapNumber::kSize, result, scratch1, scratch2, gc_required,
+           TAG_OBJECT);
+
+  // Set the map.
+  mov(FieldOperand(result, HeapObject::kMapOffset),
+      Immediate(isolate()->factory()->mutable_heap_number_map()));
+}
+
+
 void MacroAssembler::AllocateTwoByteString(Register result,
                                            Register length,
                                            Register scratch1,