Fix latest dart2js and dart2wasm failing to compile due to invalid method signatures detected (#1326)

[SleepySquash]

This PR fixes #1326 (just_audio_web issue with latest dart2js and dart2wasm compilers) by adding more strict type signature to the functions. Dart incorrectly decides that (event) is a dynamic type, thus failing to compile. Refactoring that to (Event event) fixes the issue.

If there's anything else I can or should do, please, let me know!

[SleepySquash]

Can validate the changes do indeed fix the issue on the Flutter 3.26.0-1.0.pre.152.

Without dependencies override:

Target dart2js failed: ProcessException: Process exited abnormally with exit code 1:
../../../.pub-cache/hosted/pub.dev/just_audio_web-0.4.12/lib/just_audio_web.dart:120:11:
Error: Function converted via 'toJS' contains invalid types in its function signature: 'Null Function(*dynamic*)'.
        }.toJS);
          ^
../../../.pub-cache/hosted/pub.dev/just_audio_web-0.4.12/lib/just_audio_web.dart:125:11:
Error: Function converted via 'toJS' contains invalid types in its function signature: 'Null Function(*dynamic*)'.
        }.toJS);
          ^
../../../.pub-cache/hosted/pub.dev/just_audio_web-0.4.12/lib/just_audio_web.dart:130:11:
Error: Function converted via 'toJS' contains invalid types in its function signature: '*Future<Null>* Function(*dynamic*)'.
        }.toJS);
          ^
../../../.pub-cache/hosted/pub.dev/just_audio_web-0.4.12/lib/just_audio_web.dart:136:11:
Error: Function converted via 'toJS' contains invalid types in its function signature: 'Null Function(*dynamic*)'.
        }.toJS);
          ^
../../../.pub-cache/hosted/pub.dev/just_audio_web-0.4.12/lib/just_audio_web.dart:141:11:
Error: Function converted via 'toJS' contains invalid types in its function signature: 'Null Function(*dynamic*)'.
        }.toJS);
          ^
../../../.pub-cache/hosted/pub.dev/just_audio_web-0.4.12/lib/just_audio_web.dart:146:11:
Error: Function converted via 'toJS' contains invalid types in its function signature: 'Null Function(*dynamic*)'.
        }.toJS);
          ^
../../../.pub-cache/hosted/pub.dev/just_audio_web-0.4.12/lib/just_audio_web.dart:151:11:
Error: Function converted via 'toJS' contains invalid types in its function signature: 'Null Function(*dynamic*)'.
        }.toJS);
          ^
../../../.pub-cache/hosted/pub.dev/just_audio_web-0.4.12/lib/just_audio_web.dart:157:11:
Error: Function converted via 'toJS' contains invalid types in its function signature: 'Null Function(*dynamic*)'.
        }.toJS);
          ^
../../../.pub-cache/hosted/pub.dev/just_audio_web-0.4.12/lib/just_audio_web.dart:162:11:
Error: Function converted via 'toJS' contains invalid types in its function signature: 'Null Function(*dynamic*)'.
        }.toJS);
          ^
Error: Compilation failed.
Compiling lib/main.dart for the Web...                             16.7s
Error: Failed to compile application for the Web.

WIth the override:

  just_audio_web:
    git:
      url: https://github.com/SleepySquash/just_audio
      path: just_audio_web
      ref: 1326-improve-web-methods-signature

Compiling lib/main.dart for the Web...                             58.5s
✓ Built build/web