title: Dart: Do I have to cancel Stream subscriptions and close StreamSinks?
body: <p>I know I have to cancel <code>Stream</code> Subscriptions when I no longer want to receive any events.
  Do I have to this even after I receive a 'Done' event? Or do I get memory leaks?</p>
  
  <p>What happens to Streams that are passed to <code>addStream</code> of another Stream? Are they automatically canceled? </p>
  
  <p>Same Question on the <code>StreamSink</code> side do I have to close them if the stream is already done?</p>