@babel/polyfill is deprecated, migrate to core-js@3

[Jonny Buchanan]

References:

• https://babeljs.io/blog/2019/03/19/7.4.0#core-js-3-7646-https-githubcom-babel-babel-pull-7646
• https://github.com/zloirock/core-js/blob/master/docs/2019-03-19-core-js-3-babel-and-a-look-into-the-future.md

[Jonny Buchanan]

@babel/polyfill is only being used in our Karma config, and only because it's still using PhantomJS, which has been unmaintained for a long time.

We probably need to start using core-js@3 for #550 anyway.