Review request for JDK-8060724

[Hannes Wallnoefer]

Please review JDK-8060724: ant test262parallel in Nashorn spends a 
significant amount of time after almost all the tests are run:

http://cr.openjdk.java.net/~hannesw/8060724/
This should speed up ant test262parallel significantly. On my computer 
it goes from 25 minutes down to 14 minutes.

Thanks,
Hannes