test: fix tests after V8 upgrade

- An error message changed for undefined references
- `let` is now allowed in sloppy mode
- ES2015 proxies are shipped and the `Proxy` global is now a function

PR-URL: https://github.com/nodejs/node/pull/4722


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>