I can make a downstream job with the DSL plugin for Jenkins:
https://github.com/jenkinsci/job-dsl-plugin/wiki/Job-reference#downstream
How can I make an upstream job (same thing...just specified in a difference place)?
In the UI, it's under Triggers: "Build after other projects are built".
Currently that's not possible. The Job DSL plugin generates the configuration XML used internally by Jenkins. The upstream job is not part of that XML, Jenkins generates the information at runtime. So if you specify the upstream in a job, what Jenkins does is setting the downstream in that upstream job and persists that. And that's something that the Job DSL currently can not do.