It appears since the last two releases, my utility class no longer resolves if I they are in my classpath, which I stipulate vis class path on the jobDsl step.

The utility class is just a groovy file with one class:
https://github.com/hypergig/robo-jenkins/blob/master/robo-jenkins/usr/share/jenkins/userContent/meta_jobs/libs/robo/RoboUtil.groovy

I use to be able to import it in my seed jobs, but it no longer works:

ERROR: startup failed:
/var/jenkins_home/workspace/seed/meta_jobs/seed/seed.groovy: 3: unable to resolve class robo.RoboUtil
 @ line 3, column 1.
   import robo.RoboUtil
   ^

I am pretty sure this is due to this change:

Use Groovy Shell instead of Groovy Script Engine to run DSL scripts, see Migration (#976)

https://github.com/jenkinsci/job-dsl-plugin/wiki#release-notes

But I am not sure... I don't fully understand what it means to use the shell over the engine.