vm: name anonymous functions

Name anonymous arrow function in vm module to improve readability

PR-URL: https://github.com/nodejs/node/pull/9388


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <inglor@gmail.com>
Ref: #8913