GROOVY-6511: Fix "illegal use of non virtual call" on JDK8
Remove build warning: don't use PermSize under jdk 8
GROOVY-6465: remove DGM#sort(Iterable,Comparator) because it conflicts with a JDK 8 method    * breaking change!    * removed because only introduced recently (2.2.0)
Fix tests for JDK8