With this PR, if a down event with isARepeat is observed without preceding down events for this key, this repeat event will be converted to a key down instead of key repeat, ensuring event regularity.

This irregular message sequence will be observed when the message list is filtered, such as what @cbracken and @LongCatIsLooong are working on with IME.

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 Hixie said 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.

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