ES2015 source code is identified correctly by current versions of JSHint (as of this writing, Atom jshint package has version 1.8.3). However, trying to use experimental (stage-0) language features like decorators produces error messages:

@bar
class Foo {
}

JSHint displays:

JSHint 1:1 Unexpected token ILLEGAL

This is by design. You can’t do anything about it. Don’t spend hours trying to find a setting which would enable support for the stage-0 language features – you won’t find any.

The JSHint Team explains how new language features will get added to JSHint and why stage-0 and stage-1 won’t be included, even in the foreseeable future.