title: Do I need to cancel StreamSubscriptions?
body: <p>I understand that every time I listen for an event, a new StreamSubscription object is created </p>
  
  <pre><code>element.onMouseMove.listen
  </code></pre>
  
  <p>My question is: when will be this StreamSubscription object freed from the memory? Will it linger in there until I call it's <em>cancel()</em> method? Or is it enough to remove the element/object the StreamSubscription is listening to?
  When exactly do I have to worry about memory leaks?</p>