What's problem with just using <script src=""></script>?
Here is Promise polyfill unpkg.com/bunnyjs@0.14.4/dist/polyfill-promise.mi…
Or you can just import 'bunnyjs/src/polyfills/Promise'; after npm install bunnyjs --save. But I would not recommend including and transpiling your code for every modern browser, only for IE11 you can do a backend user agent string check and add an additional <script> with all polyfills for that browser.