Through the constructor, similar to the `$util` param in my other classes.  Like this:

    public function __construct(OAuth2_Storage_JWTBearerInterface $storage, $audience, OAuth2_Util_JWT)
    {
        $this->storage = $storage;
        $this->audience = $audience;

        if (is_null($util)) {
             $util = new OAuth2_Util_JWT();
        }

        $this->util = $util;
    }