          Returns true if there is at least one key/value pair in the map.
          <div class="features">read-only</div>
</dd>
        <dt id="keys" class="property">
          <span class="name"><a href="dart-collection/MapMixin/keys.html">keys</a></span>
          <span class="signature">&#8594; <a href="dart-core/Iterable-class.html">Iterable</a><span class="signature">&lt;K&gt;</span></span>
        </dt>
        <dd>
          The keys of <a href="dart-collection/MapMixin-class.html">this</a>. <a hre