Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/objects-debug.cc

Index: src/objects-debug.cc
diff --git a/src/objects-debug.cc b/src/objects-debug.cc
index 5ca07d5add7419aee5f4e509230c8864787045ce..73c1198a8cdac4269b2d59f6dae598458a88eb1f 100644
--- a/src/objects-debug.cc
+++ b/src/objects-debug.cc
@@ -87,6 +87,7 @@ void HeapObject::HeapObjectVerify() {
       Map::cast(this)->MapVerify();
       break;
     case HEAP_NUMBER_TYPE:
+    case MUTABLE_HEAP_NUMBER_TYPE:
       HeapNumber::cast(this)->HeapNumberVerify();
       break;
     case FIXED_ARRAY_TYPE:
@@ -232,7 +233,7 @@ void Symbol::SymbolVerify() {
 
 
 void HeapNumber::HeapNumberVerify() {
-  CHECK(IsHeapNumber());
+  CHECK(IsHeapNumber() || IsMutableHeapNumber());
 }
 
 
@@ -290,7 +291,7 @@ void JSObject::JSObjectVerify() {
         Representation r = descriptors->GetDetails(i).representation();
         int field = descriptors->GetFieldIndex(i);
         Object* value = RawFastPropertyAt(field);
-        if (r.IsDouble()) ASSERT(value->IsHeapNumber());
+        if (r.IsDouble()) ASSERT(value->IsMutableHeapNumber());
         if (value->IsUninitialized()) continue;
         if (r.IsSmi()) ASSERT(value->IsSmi());
         if (r.IsHeapObject()) ASSERT(value->IsHeapObject());