Use Ubuntu-16.04 (#16)
Use Ubuntu-16.04 (#16)

Once https://chrome-internal-review.googlesource.com/c/infradata/config/+/859013 lands, we'll be able to use Ubuntu 16, which is more consistent with other parts of our Cirrus/Docker infra.

__this change should not land until https://chrome-internal-review.googlesource.com/c/infradata/config/+/859013 does!!__
flutter/infra