Review request for 8u-dev backport of JDK-8057691

[Hannes Wallnoefer]

The changes to Parser.java in JDK-8057691 did not apply cleanly to 
8u-dev because of changes in 9.  Please review manual backport:

JDK9:
http://cr.openjdk.java.net/~hannesw/8057691/webrev/src/jdk.scripting.nashorn/share/classes/jdk/nashorn/internal/parser/Parser.java.udiff.html
JDK8u
http://cr.openjdk.java.net/~hannesw/8057691/webrev-8u-dev/src/jdk/nashorn/internal/parser/Parser.java.udiff.html

The resulting code is identical (no more nested scope for top-level 
let/const).

Thanks,
Hannes