At the time of writing, some of the fullscreen APIs are marked as experimental in the Dart API documentation, and this is made clear in the documentation as well. By the time you read this, the fullscreen APIs should hopefully be stabilized and fully operational.

This is the normal process for new web browser features, which are eventually made stable. The adding of experimental APIs is likely to be a continuous process—it seems very unlikely that web browsers will ever be 100 percent complete! The onFullscreenChange Dart API documentation reads:

Experimental
ElementStream<Event> get onFullscreenChange
Stream of fullscreenchange events handled by this Element.

The source code for this part of the dart:html package has several annotations including the @experimental marker:

  /// Stream of `fullscreenchange` events handled by this [Element].
  @DomName('Element.onwebkitfullscreenchange')
  @DocsEditable()
  // https://dvcs.w3.org/hg/fullscreen/raw-file/tip/Overview.html...