MacOS: Do not send key event if modifiers flags haven't changed

[knopp]

Fixes flutter/flutter#77535

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[flutter-dashboard]

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[dkwingsmt]

Looks good, can you add a unit test?

[knopp]

@dkwingsmt, I added a test for this. I also noticed that FlutterViewControllerTest tests fail (unrelated to this PR), do you happen to know why? Is this expected?

[dkwingsmt]

It's a very weird error... can you try sync'ing with master?

[knopp]

@dkwingsmt, I rebased the PR and the checks are now passing.

[dkwingsmt]

LGTM