polyfill-regenerator pollutes global scope even if set to "usage-pure"

I might be misunderstanding here, but I thought using `polyfill-regenerator` was not supposed to pollute global scope if set to `usage-pure`. You can see a repro [here](https://github.com/utrolig/webpack-polyfills).

When I build the app and open the `index.html` file, I can still see the `regeneratorRuntime` global variable.

Start with the linked webpack-polyfills reproduction: build the app and open index.html, then inspect where regeneratorRuntime appears despite the usage-pure setting. Compare that behavior with polyfill-regenerator's usage-pure configuration; done means the build no longer exposes regeneratorRuntime globally under that setting.

Written by the indexing model from the issue text.