sync</a>.</p>
<p>There is no guarantee that an error is only sent through <a href="dart-async/Zone/errorCallback.html">errorCallback</a>
once. Libraries that use intermediate controllers or completers might
end up invoking <a href="dart-async/Zone/errorCallback.html">errorCallback</a> multiple times.</p>
<p>Returns <code>null</code> if no replacement is desired. Otherwise returns an instance
of <a href="dart-async/AsyncError-class.html">AsyncError</a> holding the new pair of error and stack trace.</p>
<p>Al