Object.assign et al are not polyfilled

Created by: conatus

Hi all,

Thanks for your wonderful work on this.

When testing a React app using create-react-app on a old version of mobile Safari I noticed that Object.assign was failing.

I looked and saw that polyfill support is turned off on the configuration of babel-plugin-transform-runtime https://github.com/facebookincubator/create-react-app/blob/master/config/babel.prod.js#L33-L37

I worked around this by, for the moment, slightly inefficiently manually adding babel-polyfill and using the more "traditional" import 'babel-polyfill'; at the root of the application to include core-js polyfills.

Seems that if the goal is zero configuration setup this is going to be a gotcha for more than a few folks. Unsure if the best approach is to simply turn on the polyfill in babel-plugin-transform-runtime as the documentation seems to suggest this is not the intent of this transformation to supplement native methods like Object.assign.