Update to core-js@3

[ianschmitz]

This is a breaking change to our 3.0 alpha that deprecates the @babel/polyfill feature in favor of the recommended approach from the babel team as part of the babel 7.4.0 release. This adds a new react-app-polyfill/stable module that can be imported from src/index.js which will include polyfills for stable language features, and integrates nicely with @babel/preset-env which uses browserslist to only include polyfills for the target browsers.

Some more relevant info on this change: https://github.com/zloirock/core-js/blob/master/docs/2019-03-19-core-js-3-babel-and-a-look-into-the-future.md#babel

Outstanding questions:

• In the corejs@2 migration guide it says

  • If you are using @babel/plugin-transform-runtime, you need to use the corejs: 3 option

  • We currently have corejs: false so my sense is we can ignore this, but it would be helpful if someone with more knowledge of this plugin could chime in

[brunolemos]

There is no react-app-polyfill/stable in the published version.

"react-app-polyfill": "1.0.0-next.b0cbf2ca",
"react-scripts": "3.0.0-next.b0cbf2ca",

[ianschmitz]

This was just merged in recently @brunolemos. We will publish a new alpha shortly.