Fix incremental compilation under Java 8
Fix incremental compilation under Java 8

This restores some code that got removed in
4d394f7bd5a94f254bb663b2b2671374506ddafd because I thought the bug was
fixed in sbt, but turns out it isn't. Without this commit, doing:
% sbt
> dotty-library-bootstrapped/compile
> exit
% sbt
> dotty-library-bootstrapped/compile

Caused dotty-library-bootstrapped to be compiled twice, because sbt
thinks the JDK has changed. This only happens in Java 8.

lampepfl/dotty,dotty-staging/dotty,som-snytt/dotty,sjrd/dotty,som-snytt/dotty,lampepfl/dotty,sjrd/dotty,sjrd/dotty,dotty-staging/dotty,som-snytt/dotty,dotty-staging/dotty,dotty-staging/dotty,lampepfl/dotty,dotty-staging/dotty,som-snytt/dotty,sjrd/dotty,som-snytt/dotty,lampepfl/dotty,sjrd/dotty,lampepfl/dotty