Buildfiles not ES5 compliant

[devatrox]

In the generated files there still is lots of ES6 syntax like arrow functions, let, const and property shorthands.

A common Webpack/Babel setup will usually ignore node_modules folder so these will not be transformed and end up causing errors on older browsers.

[PGA68]

Yes.
I have the same problem. For this reason, create-react-app does not work in Internet Explorer 11. Object.assign is not supported by this browser.

Fixed for it in my case (only production build) - facebook/create-react-app#7084 (comment)