SweetAlert in Laravel 5.5 using laravel-mix not work properly with css

I try to use SweetAlert with Laravel 5.5 usin Larvel Mix, but not work properly

webpack.mix.js

mix.js('resources/assets/js/app.js', 'public/js')
   .sass('resources/assets/sass/app.scss', 'public/css')
   .copy('node_modules/sweetalert/dist', 'public/js');

package.json

 "devDependencies": {
        ...
        "bootstrap-sass": "^3.3.7",
        ...
        "jquery": "^3.2",
        "laravel-mix": "^1.0",
        "sweetalert": "^2.0.8",

resource/assets/sass/app.css

// Sweet Alert
@import url("http://ift.tt/2BFc3uv");

when I execute npm run dev compiling without any errors.

But when I use with this method, alerts show without CSS style of SweetAlert.

via Chebli Mohamed