This is fine.
I was kind of just giving this a try. Rollup is supposed to emit a bundle that uses ES modules inside of it, and is able to be consumed by webpack, etc. just as if the tool were consuming a folder of modules. Perhaps it doesn't work well though -- was just an experiment.
My concern is that it emits a bunch of polyfill, which you don't need if you're in an environment that speaks modules. I suspect that Closure will have a better time minifying an actual class than a polyfilled/obfuscated one.