Only use OwnDescriptors to decide whether to go slow or not.

src/objects.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 1672 matching lines @@
       return value;
     } else {
       Handle<Object> args[1] = {Handle<String>(name)};
       return heap->isolate()->Throw(
           *FACTORY->NewTypeError("object_not_extensible",
                                  HandleVector(args, 1)));
     }
   }
   if (HasFastProperties()) {
     // Ensure the descriptor array does not get too big.
-    if (map_of_this->instance_descriptors()->number_of_descriptors() <
+    if (map_of_this->NumberOfOwnDescriptors() <
         DescriptorArray::kMaxNumberOfDescriptors) {
       if (value->IsJSFunction()) {
         return AddConstantFunctionProperty(name,
                                            JSFunction::cast(value),
                                            attributes);
       } else {
         return AddFastProperty(name, value, attributes, store_mode);
       }
     } else {
       // Normalize the object to prevent very large instance descriptors.
@@ skipping 11841 matching lines @@
   set_year(Smi::FromInt(year), SKIP_WRITE_BARRIER);
   set_month(Smi::FromInt(month), SKIP_WRITE_BARRIER);
   set_day(Smi::FromInt(day), SKIP_WRITE_BARRIER);
   set_weekday(Smi::FromInt(weekday), SKIP_WRITE_BARRIER);
   set_hour(Smi::FromInt(hour), SKIP_WRITE_BARRIER);
   set_min(Smi::FromInt(min), SKIP_WRITE_BARRIER);
   set_sec(Smi::FromInt(sec), SKIP_WRITE_BARRIER);
 }
 
 } }  // namespace v8::internal