the call to <code>runZoned</code> returns <code>null</code>.</p>
<p>If the zone specification has a <code>handleUncaughtError</code> value or the <code>onError</code>
parameter is provided, the zone becomes an error-zone.</p>
<p>Errors will never cross error-zone boundaries by themselves.
Errors that try to cross error-zone boundaries are considered uncaught in
their originating error zone.</p>
<pre class="language-dart"><code>var future = new Future.value(499);
runZoned(() {
  var future2 = future.then((_)