[video_player_avfoundation] Create a Dart player instance

[stuartmorgan-g]

Refactors the Dart internals to create a pleyer instance class on the Dart side, and intermediates the event stream with an instance-managed stream controller that would allow for alternate event sources (e.g., Dart code).

This prepares for future moves of more logic to Dart, and parallels Android Dart code changes made in
#9771

Part of flutter/flutter#172763

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Footnotes

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[stuartmorgan-g]

This was just an opportunistic change; I had mass-changed flutter.io to flutter.dev in most places a while ago, and apparently missed this channel name.

[gemini-code-assist]

Code Review

This pull request refactors the Dart implementation of the AVFoundation video player to introduce a _PlayerInstance class. This new class encapsulates the state and behavior for a single video player instance, including handling API calls and event streams. This is a good architectural improvement that increases modularity and maintainability. The event channel name has also been updated from flutter.io to flutter.dev for consistency. The tests have been updated to reflect these changes.

I've found a couple of issues related to the new StreamController in _PlayerInstance that could lead to resource leaks or runtime errors. Please see my detailed comments.

[gemini-code-assist]

The _eventStreamController is created as a single-subscription StreamController. This can lead to runtime errors if there are multiple attempts to listen to the videoEvents stream. It's safer to use a broadcast stream controller to allow for multiple listeners, which makes the implementation more robust.

Suggested change

	final StreamController<VideoEvent> _eventStreamController =
	StreamController<VideoEvent>();
	final StreamController<VideoEvent> _eventStreamController =
	StreamController<VideoEvent>.broadcast();

[gemini-code-assist]

The _eventStreamController is not closed in the dispose method. This can lead to a resource leak. You should close the stream controller to release its resources when the player instance is disposed.

    await _eventStreamController.close();
    await _api.dispose();