`undefined` incorrectly labeled as unused.

[jdalton]

Here is a small repro:

(function() {
  var undefined;
  function foo() {
    return arguments[0] || undefined;
  }
  return foo;
}());

[kangax]

Looks like JSHint issue. Plugged this in jshint.com and it showed "undefined" as unused.

[kangax]

Filed as https://github.com/jshint/jshint/issues/1604