Extra Copy of R8

In this directory we keep an extra copy of r8 that is infrequently updated. This copy of r8 is used exclusively for dexing, since dexing (i.e. d8) code does not change as frequently, it does not need to be rolled as often as the rest of the code in r8. Every time it is rolled, all incremental dexing on dev machines as well as bots is invalidated, which means that the next build will need to do a full dex run, significantly slowing down bots that are already slow (e.g. android-internal-rel and android-internal-dbg).

This extra copy of r8 can be updated by updating its entry in //DEPS with the CIPD instance ID of the current //third_party/r8 entry.