t of this stream.</p>
<p>Stops listening to this stream after the first element has been received.</p>
<p>Internally the method cancels its subscription after the first element.
This means that single-subscription (non-broadcast) streams are closed
and cannot be reused after a call to this getter.</p>
<p>If an error event occurs before the first data event, the returned future
is completed with that error.</p>
<p>If this stream is empty (a done event occurs before the first data event),
the returned future 