This PR addresses an issue where autocompleting a text field, even without direct keyboard input, unexpectedly triggers keydown events. To resolve this, the code now relaxes the casting assumptions to accommodate a wider range of event types, not just keyboard events.

By just adding the following script to the console, and filling the text field using autocomplete, you can see that indeed the fired event is not of type KeyboardEvent but Event.

document.body.addEventListener('keydown', console.log)

Fixes flutter/flutter#149968

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.