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

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index a98387cc325e7b39cc553aa2459089bdbdbaaddd..fb2ad7fee2122c605c4e94019138b322b8fc471a 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -2904,16 +2904,8 @@ MaybeObject* JSObject::SetPropertyForResult(LookupResult* result,
       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);
@@ -3037,6 +3029,7 @@ MaybeObject* JSObject::SetLocalPropertyIgnoreAttributes(
       Object* transition = result.GetTransitionValue();
 
       if (transition->IsAccessorPair()) {
+        ASSERT(!AccessorPair::cast(transition)->ContainsAccessor());
         return ConvertDescriptorToField(name, value, attributes);
       }