ass="desc markdown">
      <p>If an object equal to <code>object</code> is in the set, return it.</p>
<p>Checks whether <code>object</code> is in the set, like <a href="../../dart-collection/SplayTreeSet/contains.html">contains</a>, and if so,
returns the object in the set, otherwise returns <code>null</code>.</p>
<p>If the equality relation used by the set is not identity,
then the returned object may not be <em>identical</em> to <code>object</code>.
Some set implementations may not be able to implement th