Polyfill

Created by: ForbesLindesay

Polyfilling via babel doesn't work very well in my experience since you don't normally run babel against node_modules. For apps (rather than libraries) I think it's generally best to just have global polyfills.

With this in mind, we could consider just adding a script to link to https://polyfill.io/v2/docs/ as the default polyfill. It provides a polyfill based on the user agent string so is a decent future proof default.

For non-polyfill babel-runtime components, we should do as @insin suggest in https://github.com/facebookincubator/create-react-app/issues/51#issuecomment-234215045