Upgraded ecma version to 2018

Suggest by @juergba at mochajs#4164

Author: UlisesGascon

.eslintrc.yml

@@ -29,7 +29,7 @@ overrides:
       - test/integration/helpers.js
       - lib/growl.js
     parserOptions:
-      ecmaVersion: 2017
+      ecmaVersion: 2018
     env:
       browser: no