First take on custom call generators.

src/factory.cc

 // Copyright 2006-2008 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 848 matching lines @@
       array = Factory::CopyAppendCallbackDescriptors(array, props);
     }
     Handle<Object> parent = Handle<Object>(obj->parent_template());
     if (parent->IsUndefined()) break;
     obj = Handle<FunctionTemplateInfo>::cast(parent);
   }
   if (!array->IsEmpty()) {
     map->set_instance_descriptors(*array);
   }
 
+  ASSERT(result->shared()->IsApiFunction());
   return result;
 }
 
 
 Handle<MapCache> Factory::NewMapCache(int at_least_space_for) {
   CALL_HEAP_FUNCTION(MapCache::Allocate(at_least_space_for), MapCache);
 }
 
 
 static Object* UpdateMapCacheWith(Context* context,
@@ skipping 77 matching lines @@
     Execution::ConfigureInstance(instance,
                                  instance_template,
                                  pending_exception);
   } else {
     *pending_exception = false;
   }
 }
 
 
 } }  // namespace v8::internal