fix nashorn support about fext

[glathoud]

This was a regression. See also:

#8

https://bugs.java.com/bugdatabase/view_bug.do?bug_id=JDK-8205600

[glathoud]

Note about nashorn: utility limited in the end for build (but okay for testing) because no let/const support at the moment (2018-07), and let/const support needed for safe inlining.

Digging into this:

 jjs -v
nashorn 1.8.0_171

let, const:
http://openjdk.java.net/jeps/203

https://bugs.openjdk.java.net/browse/JDK-8046038

[glathoud]

jjs --language=es6

adds support for let/const

[glathoud]

...and now this:
http://openjdk.java.net/jeps/335
apparently Oracle is too small of a company to support Nashorn. Interesting...