Please don't set these variables globally.

Instead, use a local `let` binding to temporarily set these variables.  Take a look at the test case `flycheck/javascript-jshint-syntax-error`, to see how [I deal with variables from js2 and js3 mode](https://github.com/flycheck/flycheck/blob/master/test/builtin-checkers-test.el#L511). Don't forget to add [corresponding `defvar`s](https://github.com/flycheck/flycheck/blob/master/test/builtin-checkers-test.el#L32).