Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/objects-printer.cc

Index: src/objects-printer.cc
diff --git a/src/objects-printer.cc b/src/objects-printer.cc
index 518167cc51e0d63216056f51b5508fdd94424b0e..e16a19664f71708cd64cdacf72fdba3b61e0e445 100644
--- a/src/objects-printer.cc
+++ b/src/objects-printer.cc
@@ -92,6 +92,11 @@ void HeapObject::HeapObjectPrint(FILE* out) {
     case HEAP_NUMBER_TYPE:
       HeapNumber::cast(this)->HeapNumberPrint(out);
       break;
+    case MUTABLE_HEAP_NUMBER_TYPE:
+      PrintF(out, "<mutable ");
+      HeapNumber::cast(this)->HeapNumberPrint(out);
+      PrintF(out, ">");
+      break;
     case FIXED_DOUBLE_ARRAY_TYPE:
       FixedDoubleArray::cast(this)->FixedDoubleArrayPrint(out);
       break;