Using modern format forces ALL outputted bundles to be modern code

[snapwich]

I think this is a bug unless I'm misunderstanding the purpose of specifying multiple output formats.

If you run microbundle --format modern,umd then both the modern.js and umd.js files that are output will not polyfill code using class, async/await, etc.

If you run microbundle --format umd then the umd.js file will be polyfilled and remove modern code like class or async/await, as I would expect being a safe default.

If this is indeed buggy behavior then I think this is a big problem because modern is a default format, which means if you don't specify a format all of your output bundles will be expected to only run in modern browsers.

[developit]

This should be fixed by #570.