[philipwalton]

You're right about it not working for webpack dev server, and I agree that it's unfortunate, but I think webpack will evolve to accommodate this.

[EDIT: it looks like this is already supported, here's the documentation explaining how to do it: https://webpack.js.org/configuration/configuration-types/#ex...]

Service Worker already introduced the need (or you could argue possibility) to have multiple configs since SW code has a much different transpiling baseline compared to legacy browser code. And module code just adds one more level to this.

I think webpack dev server will update if this practice becomes popular, but for the moment I think your idea of only building the es2015 build in dev mode is probably the best temporary solution (as long as you make sure to run your test suite in multiple environments and include the legacy build there).