This allows all leaf data sources (i.e. ones which do not forward the requests
to other data sources) to accept multiple listeners.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=203097587
These don't seem to be needed anymore. All tests run without them in gradle
and Blaze.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=191867518