ReferenceUnion is a hack to work around the mismatch between how
we store references (by roid) and how they are represented in EPP
(by foreign key). If it ever needed to exist (not entirely clear...)
it should have remained tightly scoped within the domain commands
and resources. Instead it has leaked everywhere in the project,
causing lots of boilerplate. This CL hides all of that behind
standard Refs, and should be followed by work to remove ReferenceUnion
completely.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=122424416
9a2afc7a9b