r <code>hashCode</code> are omitted, the set uses
the elements' intrinsic <a href="dart-core/Object/operator_equals.html">Object.==</a> and <a href="dart-core/Object/hashCode.html">Object.hashCode</a>.</p>
<p>If you supply one of <code>equals</code> and <code>hashCode</code>,
you should generally also to supply the other.</p>
<p>If the supplied <code>equals</code> or <code>hashCode</code> functions won't work on all <code>E</code>
objects, and the map will be used in a setting where a non-<code>E</code> obj