In https://git.eclipse.org/r/113120 I have some changes which should
hopefully produce our first successful build with compiler tests running
on Java 9 and thus executing also 9-only tests.
If that job succeeds, feel free to release the gerrit.
Otherwise, s.o. may have to disable the new maven profile in the jenkins
configuration, to allow successful builds on Java 8 still:
Just remove "-Ptest-on-javase-9".
The other interim tweak I made in the configuration (create toolchains.xml
on the fly and refer mvn to it) shouldn't harm when going back to Java 8.
(to be removed when https://bugs.eclipse.org/528366 is resolved).