v2.2 mongodump is incompatible with pre-v2.2 mongorestore

mongodump's file layout is different in v2.2 than in v2.0 in a way that will lead to undesirable results if somebody happens to run a v2.0 mongorestore on a v2.2 mongodump layout. In particular, the new mongodump doesn't produce a system.indexes.bson for each database, and old mongorestore doesn't know what to do with the <collection>.metadata.json, so restoring with an old mongorestore will not retain indexes that existed in the dumped database.

If we want to call this a regression, then a fix would be to have mongodump additionally create the system.indexes.bson file.

Alternatively we could call this a documentation bug and update the release notes.