RFR 8075207: Nashorn parser API returns StatementTree objects in out of order

[A. Sundararajan]

Please review http://cr.openjdk.java.net/~sundar/8075207/ for 
https://bugs.openjdk.java.net/browse/JDK-8075207

PS. Piggybacking few @Override and other clean-ups..
Thanks,
-Sundar