`HtmlDocument`'s `visibilityState` is `String?` when compiling tests.

[harkairt]

Please refer to this repository: https://github.com/harkairt/test_compile_error
especially this line

Steps:

• clone the repository
• fvm use (or simply use a global flutter 3.16.5)
• flutter pub get && flutter run -d 'chrome'
  • works well, the app runs, and if you switch to a new tab, the proper message is written to debug console
• flutter test
  • ❌ Error: Method 'toTabVisibilityChangeEvent' cannot be called on 'String?' because it is potentially null. Try calling using ?. instead.
  • if you switch out the commenting of these two lines

      final event = document.visibilityState.toTabVisibilityChangeEvent();
      // final event = (document.visibilityState ?? '').toTabVisibilityChangeEvent();

• then executing the test again will succeed ✅

note: the IDE will warn about unnecessary fallback '' since it recognizes visibilityState to be String and not String?