This was a surprisingly involved change. Some of the difficulties included
java.util.Optional purposely not being Serializable (so I had to move a
few Optionals in mapreduce classes to @Nullable) and having to add the Truth
Java8 extension library for assertion support.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=171863777
While I am doing this, promote LoadAndResaveEntityCommand from
javascrap to the main tool since it's repeatedly been useful, and
rename it and update its documentation to better reflect the
difference between that command and the one I am adding.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=139460841