Set up Babel polyfills for browser compatibility

[colinrotherham]

What

Set up Babel to automatically add polyfills for unsupported browser APIs.

Ideally, unsupported code should be replaced with "pure" polyfills (ponyfills) that do not modify browser globals.

Why

This issue follows on from #3504 and is only needed when we choose to use a browser API unavailable in .browserlistrc supported browsers

Polyfill code should not cause parsing or syntax errors. Since feature detection can guard against code we choose not to run, unsupported browser APIs don't have to be polyfilled.

This will be done with caution while keeping an eye on the impact of the polyfills on the size of the built files.

Who needs to work on this

Developers

Who needs to review this

Developers

Depends on

• Set up Babel transpilation for browser compatibility #3504

Done when

Beta Give feedback

1. We've reviewed the relevance of Use polyfills from npm package #3082
2. We've picked a polyfill library like core-js
3. Keep tracking impact of polyfills once automated #3283
   javascript performance tooling +3
4. Update our internal documentation on polyfilling #3722
   documentation javascript +2