use elixir.mixins in a task extend

in laravel/elixir 2 I can use like this

elixir.extend('dump', function () {
    var mix = this;
    gulp.task('dump', function () {
        if (argv.thm) {
            var thm = argv.thm.split(".");
            var configs = mix.thmConfigs().themes();
            mix.thmDump(configs[thm[0]][thm[1]]);
        } else {
            mix.thmDumpAll();
        }
    });
    return this.queueTask('dump');
});

but I get the error in version 3 so,when I see the code with elixir3,I try this

elixir.extend('dump', function() {
    new Task('dump', function() {
        elixir.mixins.sass('app.scss');
    });
});

but it not work,how I can solving this problem,who can help me,thanks!