handle @babel/polyfill deprecation

Created by: sqs

This was approved in https://github.com/sourcegraph/sourcegraph/pull/3954, but I accidentally merged it into a non-master branch instead of rebasing before merging.

Fixes the warning:

`@babel/polyfill` is deprecated. Please, use required parts of `core-js` and `regenerator-runtime/runtime` separately

See https://github.com/zloirock/core-js/blob/master/docs/2019-03-19-core-js-3-babel-and-a-look-into-the-future.md#babelpolyfill for more information.