Add DRE to Polyfill.io service

[jfbrennan]

The wonderful Polyfill.io service doesn't have a CE polyfill. Have you considered adding this polyfill to their library? People have already suggested it https://github.com/Financial-Times/polyfill-service/pull/1376#issuecomment-369896743

[WebReflection]

I like the idea behind Polyfill.io service, and it could definitively provide custom elements and built-in extends there, targeting either modern WebKit browsers for the latter, or every older browser for the first case, where customElements is not defined.

However, custom elements built-in extend polyfill has its own caveats, so that if you don't know your browser is being polyfilled with such caveats, the DX might be bad (i.e. no constructor).