If you don't need to extract CSS that is imported in JS files (which I dislike anyway), you can just use the TypeScript compiler that will work with ES6 modules

If you don't need to extract CSS that is imported in JS files (which I dislike anyway), you can just use the TypeScript compiler that will work with ES6 modules and transpire into as low as ES3. And it has 0 dependency. Unfortunately it does not bundle to usable file for browsers, but SystemJS and AMD, and then we are back to RollUp or Parcel or Webpack. One day...