Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/objects-printer.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 74 matching lines @@
   switch (instance_type) {
     case SYMBOL_TYPE:
       Symbol::cast(this)->SymbolPrint(out);
       break;
     case MAP_TYPE:
       Map::cast(this)->MapPrint(out);
       break;
     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;
     case CONSTANT_POOL_ARRAY_TYPE:
       ConstantPoolArray::cast(this)->ConstantPoolArrayPrint(out);
       break;
     case FIXED_ARRAY_TYPE:
       FixedArray::cast(this)->FixedArrayPrint(out);
       break;
     case BYTE_ARRAY_TYPE:
@@ skipping 1125 matching lines @@
     }
   }
   PrintF(out, "\n");
 }
 
 
 #endif  // OBJECT_PRINT
 
 
 } }  // namespace v8::internal