Remove unnecessary browser polyfills

[mshustov]

I was testing what performance benefits we can gain by migrating to TS v4.2 WIP is here https://github.com/restrry/kibana/tree/ts-4-2-rc
One of the errors was caused by ResizeObserver type incompatibility in lib.d.ts and resize-observer-polyfill package.
I went to http://caniuse.com to check whether we need this polyfill for our browser support matrix
It seems that we can remove:

• resize-observer-polyfill https://caniuse.com/resizeobserver
• abort-controller-polyfill (for browsers, we still need it for nodejs) https://caniuse.com/abortcontroller [kbn-ui-shared-deps] Remove outdated polyfills #96339
• custom-event-polyfill https://caniuse.com/customevent [kbn-ui-shared-deps] Remove outdated polyfills #96339
• childnode-remove-polyfill https://caniuse.com/childnode-remove [kbn-ui-shared-deps] Remove outdated polyfills #96339

Not sure that we can get rid of regenerator-runtime/runtime. According to the comment, it's required for EUI

kibana/packages/kbn-test/src/jest/babel_transform.js

Line 17 in 4584a8b

// disable built-in filtering, which is more performant but strips the import of `regenerator-runtime` required by EUI

nevertheless, the comment might be outdated.

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)

[mshustov]

ResizeObserver typings are added to lib.d.ts microsoft/TypeScript#28502 (comment)

@jbudz Is it the only reason to keep ResizeObserver polyfill?

[jbudz]

The other reason is it's used in development for test mocking here. We'll have to replace the mock implementation.

[mshustov]

We'll have to replace the mock implementation.

We already have a mock https://github.com/elastic/kibana/blob/master/src/plugins/kibana_utils/public/resize_checker/resize_checker.test.ts#L43

We just need to mock native API instead of polyfill. Don't we?

jest.spyOn(window, 'ResizeObserver').mockImplementation(() => {
  // the same code as in the test file
})

[Dosant]

Didn't see this issue before creating this specifically for resize-observer-polyfill #121509

I think we can remove the polyfill ? I listed all places it is used and tagged relevant teams

[mshustov]

@Dosant it might be easier to remove all the places at once. I'm closing the current issue, let's use the one you created to track the effort