These have been here for a long time and appear to be harmless.
Long-term, I think we should work to not use Scala for tox4j, but that's
future me's problem.
atox/proguard-tox4j.pro
added: 6, removed: 1, total 5
@@ -9,3 +9,8 @@
-keep public class scala.collection.mutable.Builder* {