scala/scala — pull request #2681 by soc (2013-06-25 00:24:42+00:00)
Title: SI-7590 TreeSet should fail fast if Ordering is null

While migrating scala.tools.nsc.util.TreeSet to
scala.collection.mutable.TreeSet, I messed up initialization order
and realized that TreeSet accepts null as an Ordering and only fails
much later.

This change makes mutable.TreeSet and immutable.TreeSet fail
immediately.