s
until the future is completed before continuing with the next element.</p>
<p>Returns a <a href="dart-async/Future-class.html">Future</a> that completes with <code>null</code> when all elements have been
processed.</p>
<p>Non-<a href="dart-async/Future-class.html">Future</a> return values, and completion-values of returned <a href="dart-async/Future-class.html">Future</a>s,
are discarded.</p>
<p>Any error from <code>action</code>, synchronous or asynchronous,
will stop the iteration and be reported in the