I noticed this as well and posted about it here:
http://forum.inductiveautomation.com/t/bug-446-runprepquery-no-longer-returning-dataset-from-script-transform-as-of-8-0-16
In the meantime you should be able cast it to a BasicDataset to retain the same functionality you had in 8.0.13.
from com.inductiveautomation.ignition.common import BasicDataset
return BasicDataset(system.db.runNamedQuery(args))