Add support for cancelling fetch requests with AbortController

[janicduplessis]

Summary

This adds https://github.com/mysticatea/abort-controller to polyfill 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

Changelog

[General] [Added] - Add support for cancelling fetch requests with AbortController

Test Plan

Added a RNTester example of cancelling fetch requests.

[pull-bot]

	Warnings
⚠️	🔒 package.json - Changes were made to package.json. This will require a manual import by a Facebook employee.

Generated by 🚫 dangerJS against a38e50c

[matthargett]

once this (and its prerequisite PR) is merged, I'd like to propose it for backport to 0.59. @dysonpro fixed this in our apps by injecting/overriding RN's fetch with yetch (thanks @jayphelps !) and some other bits of magic.

we'd just need to make sure this doesn't introduce the same memory leak issues like the last time this abort controller support was attempted.

[janicduplessis]

@matthargett What memory leak are you talking about? Is it the one I described in #24418 or something else I should be aware of?

[cpojer]

@matthargett we aren't going to backport new features to 0.59, let's keep this work focused on 0.60+.

[cpojer]

@janicduplessis thank you so much, this is so good!

[facebook-github-bot]

@cpojer is landing this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[react-native-bot]

This pull request was successfully merged by @janicduplessis in 5e36b0c.

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