rns <code>null</code>.</p>
<p>Any object can be used as key, as long as it has compatible <code>operator ==</code>
and <code>hashCode</code> implementations.
By controlling access to the key, a zone can grant or deny access to the
zone value.</p>
    </section>
    
    <section class="summary source-code" id="source">
      <h2><span>Implementation</span> </h2>
      <pre class="language-dart"><code class="language-dart">operator [](Object key);</code></pre>
    </section>

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