Summary:
This adds https://github.com/mysticatea/abort-controller to polyfill [AbortController](https://developer.mozilla.org/en-US/docs/Web/API/AbortController). This is used to cancel requests when using `fetch`.
This also updates `event-target-shim` to 5.0 to make sure we only have one version of this dependency. This updates required adding a polyfill for `console.assert` which is used by the new version. I made one based on https://github.com/gskinner/console-polyfill/blob/master/console.js#L74.
The polyfill is very small, especially since we already use `event-target-shim` so I think it makes sense to include in core.
Depends on #24418 so that the fetch polyfill supports the `signal` parameter.
Fixes#18115
[General] [Added] - Add support for cancelling fetch requests with AbortController
Pull Request resolved: https://github.com/facebook/react-native/pull/24419
Differential Revision: D14912858
Pulled By: cpojer
fbshipit-source-id: 8a6402910398db51e2f3e3262f07aabdf68fcf72
5e36b0c6eb