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Co-authored-by: Kate Lovett <katelovett@google.com>
Co-authored-by: LongCatIsLooong <31859944+LongCatIsLooong@users.noreply.github.com>
## Description
This PR makes DropdownMenu rematching the initialSelection when the entries are updated.
If the new entries contains one entry whose value matches `initialSelection` this entry's label is used to initialize the inner text field, if no entries matches `initialSelection` the text field is emptied.
## Related Issue
Fixes [DropdownMenu.didUpdateWidget should re-match initialSelection when dropdownMenuEntries have changed](https://github.com/flutter/flutter/issues/155660).
## Tests
Adds 3 tests.
