Restriction of module redefineability in recent commit

[Rafael Winterhalter]

I just found out about this change to restrict redefining some modules:
http://hg.openjdk.java.net/jigsaw/jake/hotspot/rev/52cdbd4a5999 and I can
only repeat my warning that this further restriction will make it harder to
migrate to Java 9 and might break tools in the process. It does neither
seem to me that a good alternative is offered if opening core modules is
required to make a Java agent work.

Is this change intending to freeze classes of java.base or to restrict
exporting? If so, does this also affect the command line to change the
properties of certain modules? Was an evaluation done how this change might
impact existing tools?

Thanks for explaining this change.
Best regards, Rafael