jigsaw/m2 is now read-only

[Alan Bateman]

Now that the initial change-sets for JEP 220 are in JDK 9 we have made 
the jigsaw/m2 for forest read-only. There are a number of open issues 
(listed in the JEP) and there will other bugs and issues. Fixed for 
those that go in via jdk9/dev.

-Alan.