deal with -Xfatal-warnings for Scala 2.12.2 release, and after

[SethTisue]

once scala/scala#5402 is merged, some projects in the community build that have -Xfatal-warnings will start failing — we've already seen that in the test runs on that PR.

What I have time to do right now (meaning in time for 2.12.2) is flip a big switch and temporarily disable -Xfatal-warnings community-build wide. At least, I think I can fairly-easily do that in dbuild.

We'll eventually want to flip that switch back, but hopefully the project maintainers will fix their projects themselves if we notify them via @-mention once 2.12.2 is released, and so that after a month or two has passed, flipping the switch back will be easy.

cc @lrytz @som-snytt

[SethTisue]

it looks like #521 will be sufficient here, no big hammer needed

[som-snytt]

When everything looks like a big nail, one always reaches for a big hammer.

[SethTisue]

btw I'm working on fixing the warnings in scala-partest

[som-snytt]

I have a fix for partest, since I was also touching another partest fix.

[SethTisue]

you snooze you lose scala/scala-partest#83

[som-snytt]

Gah, conflicts.