Reconsider bundled polyfills

Created by: gaearon

See my comment in https://github.com/facebookincubator/create-react-app/pull/3200#issuecomment-340219166. We should remove any polyfills that are only necessary for browsers that aren't supported out of the box by React 16 anyway. We should also document the most efficient way polyfill for the lowest possible supported browsers by React 16 (AFAIK IE9). This would be a breaking change.