[isabelle-dev] scala-2.12.2

[Lars Hupel]

> With Isabelle/a43a079156a6 we are on scala-2.12.2, leaving the 2.11.x
> behind.

See also Isabelle/d76937b773d9, which repairs a broken code adaptation.
Note that `error` had been deprecated for at least one major release cycle.

Maybe we should start performing code checks with stricter compile flags
(e.g. "fatal warnings") to detect these problems early on.

Additionally, we might want to investigate checking multiple versions of
target languages. Three of the four platforms (Scala, Haskell, OCaml)
have tooling for selecting specific compiler versions available (sbt,
Stack, OPAM, respectively), whereas the fourth platform (SML) hardly, if
ever, makes breaking changes.

Cheers
Lars