nError</code> is not given, and this future completes with an error,
the error is forwarded directly to the returned future.</p>
<p>In most cases, it is more readable to use <a href="dart-async/Future/catchError.html">catchError</a> separately, possibly
with a <code>test</code> parameter, instead of handling both value and error in a
single <a href="dart-async/Future/then.html">then</a> call.</p>
<p>Note that futures don't delay reporting of errors until listeners are
added. If the first <code>then</code> o