Review request for JDK-8057691: Nashorn: let & const declarations are not shared between scripts

[Hannes Wallnoefer]

Please review JDK-8057691: Nashorn: let & const declarations are not 
shared between scripts:

http://cr.openjdk.java.net/~hannesw/8057691/

This implements global top-lexical scope as per the latest ES6 spec. 
When running with --language=es6 this will add a switchpoint to 
non-constant global scope properties that will be invalidated when a 
script adds global lexical bindings. I've tested with Octane and there 
are no performance regressions when running with --language=es6 compared 
to default settings.

Hannes