Fix build error in flutter beta channel caused by "function converted via 'toJS' contains invalid types"

[zhuhaichao518]

The latest beta channel does not support without type annotations like

        (event) {
          _durationCompleter?.completeError(_audioElement.error!);
        }.toJS);

It raises error with

Error: Function converted via 'toJS' contains invalid types in its function signature: 'Null Function(*dynamic*)'.
        }.toJS);

Added event type for them.

same issue: ryanheise/just_audio#1326

[cloudwebrtc]

lgtm!