[8u60] approval request for 8072595: nashorn should not use obj.getClass() for null checks

[A. Sundararajan]

Please approve.

bug: https://bugs.openjdk.java.net/browse/JDK-8072595
8u60 webrev: http://cr.openjdk.java.net/~sundar/8072595/8u60/webrev.00/
jdk9 review thread: 
http://mail.openjdk.java.net/pipermail/nashorn-dev/2015-February/004148.html

jdk8u-dev changes is exactly same as that of jdk9 except for modular 
source layout difference.

Thanks,
-Sundar