Use mutable heapnumbers to store doubles in fields.

src/string-stream.cc

Index: src/string-stream.cc
diff --git a/src/string-stream.cc b/src/string-stream.cc
index 61eec0d696d52fa10a0a439d38fdfd8640efa936..e29d5d4a0f8989e1fbad1754083ed7b977502082 100644
--- a/src/string-stream.cc
+++ b/src/string-stream.cc
@@ -368,7 +368,7 @@ void StringStream::PrintUsingMap(JSObject* js_object) {
           key->ShortPrint();
         }
         Add(": ");
-        Object* value = js_object->FastPropertyAt(descs->GetFieldIndex(i));
+        Object* value = js_object->RawFastPropertyAt(descs->GetFieldIndex(i));
         Add("%o\n", value);
       }
     }