where does the FLUSH script binding variable get set?

[Justin Edelson]

While troubleshooting SLING-1941, I noticed that it doesn't seem like the
FLUSH key is set. It's definitely not happening explicitly inside
DefaultSlingScript.verifySlingBindings(). And it isn't happening the
service() either.

Is the intent that FLUSH is set by the script itself?

Thanks,
Justin