-d=<dir> fails for any dir but . in Nashorn

e.g.


alhazred:make marcus [b102]$ java -jar ../dist/nashorn.jar -co -d=/tmp/ ../test/script/basic/NASHORN-376.js


fails with
[compiler] Skipping class dump for jdk/nashorn/internal/scripts/Script$NASHORN_376: cannot write "jdk/nashorn/internal/scripts"

while -d=. works fine