ll objects <code>o1</code>, <code>o2</code>, and <code>o3</code>, if <code>o1 == o2</code> and
<code>o2 == o3</code> are true, then <code>o1 == o3</code> must be true.</p>
</li>
</ul>
<p>The method should also be consistent over time,
so whether two objects are equal should only change
if at least one of the objects was modified.</p>
<p>If a subclass overrides the equality operator it should override
the <a href="dart-ui/CallbackHandle/hashCode.html">hashCode</a> method as well to maintain consistency.</p>
