lib: refactor wrap_js_stream for ES6/readability

PR-URL: https://github.com/nodejs/node/pull/16158


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>