alled with just the error object.
If <code>onError</code> is omitted, any errors on this stream are considered unhandled,
and will be passed to the current <a href="dart-async/Zone-class.html">Zone</a>'s error handler.
By default unhandled async errors are treated
as if they were uncaught top-level errors.</p>
<p>If this stream closes and sends a done event, the <code>onDone</code> handler is
called. If <code>onDone</code> is <code>null</code>, nothing happens.</p>
<p>If <code>cancelOnError</code> is true, 