fix KeySet.hashCode; enable multiple web tests

This is required to roll forward package:vm_service to 3.0.0 and is
blocking a DevTools release.

This fixes a problem where if you press "Shift" and then "A", then release "Shift" and then "a", then the "A" key will be "stuck" on because the logical key for the key down message is different (capital "A") from the logical key for the key up message (lowercase "a").

This PR changes the pressed keys logic so that it uses the physical key to add/remove keys from the list of pressed keys, but keeps the associated logical key.

This does mean that after the "Shift" key goes up, the pressed keys contains a capital "A" and it doesn't switch to be a lowercase "a", but there isn't currently any mechanism we can use to do that remapping. This is far less surprising than the current behavior, but is still not quite correct.

I fixed the event simulation code to take a physicalKey so that it could be matched with the logical key, but the event simulation code isn't up to the task, since it can only simulate keys that appear in the key maps. The new platform key event design should fix that (added TODOs).

This PR is a follow up of https://github.com/flutter/flutter/pull/42640
Some changes of that PR includes redundant changes using FlutterError.fromParts constructor even though it's not necessary.

Some minor changes are:

- Remove one unnecessary todo
- Fix indent consistencies

This reverts commit d3387bc9.

This reverts commit e03f4391.

[a11y] Make sure RenderFractionalTranslation updates its semantics after the translation field is set (#48985)

Add TargetPlatform.macOS tests to cupertino, foundation, rendering, and services libraries. (#45644)

This adds the ability to define variants of tests with different environmental values for a particular testWidgets test.

This allows you to run the same test multiple times with a different test environment. One test variant has been implemented that allows running a test with different settings of the TargetPlatform.