When we find an accessor pair as TRANSITION, we are sure it doesn't contain an accessor yet.

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 2886 matching lines @@
     }
     case INTERCEPTOR:
       return self->SetPropertyWithInterceptor(*name,
                                               *value,
                                               attributes,
                                               strict_mode);
     case TRANSITION: {
       Object* transition = result->GetTransitionValue();
 
       if (transition->IsAccessorPair()) {
-        if (!AccessorPair::cast(transition)->ContainsAccessor()) {
-          return self->ConvertDescriptorToField(*name,
-                                                *value,
-                                                attributes);
-        }
-        return self->SetPropertyWithCallback(transition,
-                                             *name,
-                                             *value,
-                                             result->holder(),
-                                             strict_mode);
+        ASSERT(!AccessorPair::cast(transition)->ContainsAccessor());
+        return ConvertDescriptorToField(*name, *value, attributes);
       }
 
       Map* transition_map = Map::cast(transition);
       DescriptorArray* descriptors = transition_map->instance_descriptors();
       int descriptor = descriptors->LastAdded();
       PropertyDetails details = descriptors->GetDetails(descriptor);
       ASSERT(details.type() == FIELD || details.type() == CONSTANT_FUNCTION);
 
       if (details.type() == FIELD) {
         if (attributes == details.attributes()) {
@@ skipping 103 matching lines @@
       attributes = result.GetAttributes();
       return ConvertDescriptorToField(name, value, attributes);
     case CALLBACKS:
     case INTERCEPTOR:
       // Override callback in clone
       return ConvertDescriptorToField(name, value, attributes);
     case TRANSITION: {
       Object* transition = result.GetTransitionValue();
 
       if (transition->IsAccessorPair()) {
+        ASSERT(!AccessorPair::cast(transition)->ContainsAccessor());
         return ConvertDescriptorToField(name, value, attributes);
       }
 
       Map* transition_map = Map::cast(transition);
       DescriptorArray* descriptors = transition_map->instance_descriptors();
       int descriptor = descriptors->LastAdded();
       PropertyDetails details = descriptors->GetDetails(descriptor);
       ASSERT(details.type() == FIELD || details.type() == CONSTANT_FUNCTION);
 
       if (details.type() == FIELD) {
@@ skipping 10262 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