First take on custom call generators.

src/factory.cc

Index: src/factory.cc
diff --git a/src/factory.cc b/src/factory.cc
index 8d207496418e99b12f0537d9ffc0ec66a77e6205..993e2b6ce9da855f1bceea4a70d36e0d46bf0f34 100644
--- a/src/factory.cc
+++ b/src/factory.cc
@@ -866,6 +866,7 @@ Handle<JSFunction> Factory::CreateApiFunction(
     map->set_instance_descriptors(*array);
   }
 
+  ASSERT(result->shared()->IsApiFunction());
   return result;
 }