Fix LoadFieldByIndex to take mutable heap-numbers into account.

src/handles.cc

Index: src/handles.cc
diff --git a/src/handles.cc b/src/handles.cc
index 398a68265cdf1c65d2b98a4d459ab972bbe34cef..92a6dec8311020493461f2de0d1dbf19e305e76b 100644
--- a/src/handles.cc
+++ b/src/handles.cc
@@ -689,6 +689,8 @@ Handle<FixedArray> GetEnumPropertyKeys(Handle<JSObject> object,
             if (field_index >= map->inobject_properties()) {
               field_index = -(field_index - map->inobject_properties() + 1);
             }
+            field_index = (field_index << 1)
+                | details.representation().IsDouble();
             indices->set(index, Smi::FromInt(field_index));
           }
         }