Bug#852645: dafny: FTBFS

[Benjamin Barenblat]

1.9.8 builds fine, but that’s a 30,000-plus-LoC diff from 1.9.7, so
trying to get the new version into Stretch seems unlikely at this
point. I suspect cherry-picking upstream’s
dc724533490f66bb553d71be1b971f2008318adc and
e7cfcf3eb135c378d12c04f35d9f85f6cb241467 commits into 1.9.7 would be
sufficient, but I don’t have time to check right now. I’ll continue to
investigate and attempt to backport a fix.