title: How to open a PopupMenuButton?
body: <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>