@ColemanCDA has developed an Android app using only FoundationEssentials, but RunLoop is only in Foundation, seemingly implemented in C by CoreFoundation. To use it, he would be forced to link against all of Foundation for that one Swift class, hence his initial question of how to "link only against FoundationEssentials."
Oh, that is annoying. Please can someone file a Github Issue asking for RunLoop to be in FoundationEssentials instead? I think that's likely the right fix here.
AFAIK, all of the basic concurrency primitives are unavailable in FoundationEssentials, which is a bummer and basically forces you to roll your own.
I seem to not be able to find the supporting documentation, but unfortunately, it isn't a simple "move from Foundation to FoundationEssentials". The components are slowly being migrated with a proper rewrite, removing the CF dependency in the process. This is required to re-core Foundation via swift-foundation. Perhaps @tinaliu or @Tony_Parker can point out the documentation for the migration plan.