tools: enable ES2017 syntax support in ESLint

This allows us to use async functions.

PR-URL: nodejs#11211
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>

.eslintrc.yaml

@@ -3,7 +3,7 @@ env:
   es6: true
 
 parserOptions:
-  ecmaVersion: 2016
+  ecmaVersion: 2017
 
 rules:
   # Possible Errors