A test case for a recent LUB progression.

A test distilled from a Lift example that compiles correctly
under 2.10.1, but not under 2.10.0.

I pinpointed the progression to:

  https://github.com/scala/scala/commit/a06d31f6#L0R6611

Chalk up another win for `dealiasWiden`.
Merge pull request #2600 from retronym/topic/lub-dealias-widen

A test case for a recent LUB progression.