nError</code> handler is used <em>both</em> to handle asynchronous errors
by overriding <a href="dart-async/ZoneSpecification/handleUncaughtError.html">ZoneSpecification.handleUncaughtError</a> in <code>zoneSpecification</code>,
if any, <em>and</em> to handle errors thrown synchronously by the call to <code>body</code>.</li>
</ul>
<p>If an error occurs synchronously in <code>body</code>,
then throwing in the <code>onError</code> handler
makes the call to <code>runZone</code> throw that error,
and otherwise 