Stream of keymessage events handled by this MediaElement.
Source
@DomName('HTMLMediaElement.onwebkitkeymessage')
@DocsEditable()
// https://dvcs.w3.org/hg/html-media/raw-file/eme-v0.1/encrypted-media/encrypted-media.html#dom-keyadded
@Experimental()
ElementStream<MediaKeyEvent> get onKeyMessage => keyMessageEvent.forElement(this);