How to open a PopupMenuButton? : <p>How do I open a popup menu from a second widget?</p>

<pre><code>final button = new PopupMenuButton(
    itemBuilder: (_) =&gt; &lt;PopupMenuItem&lt;String&gt;&gt;[
          new PopupMenuItem&lt;String&gt;(
              child: const Text('Doge'), value: 'Doge'),
          new PopupMenuItem&lt;String&gt;(
              child: const Text('Lion'), value: 'Lion'),
        ],
    onSelected: _doSomething);

final tile = new ListTile(title: new Text('Doge or lion?'), trailing: button);
</code></pre>

<p>I want to open the <code>button</code>'s menu by tapping on <code>tile</code>.</p>
