[8u] approval request 8136694: Megemorphic scope access does not throw ReferenceError when property is missing

[Sundararajan Athijegannathan]

bug: https://bugs.openjdk.java.net/browse/JDK-8136694
jdk9 review thread: 
http://mail.openjdk.java.net/pipermail/nashorn-dev/2015-September/005278.html
jdk8u webrev: http://cr.openjdk.java.net/~sundar/8136694/8u/webrev.00/

Backported 'as is' except for modular source layout difference.

Thanks,
-Sundar