title: How to set DropdownButton's value programmatically?
body: <p>For example, in order to set the text in a <code>TextFormField</code>, I can use a <code>TextEditingController</code>:</p>
  
  <pre><code>textEditingController = TextEditingController()
  ...
  
  TextFormField(
    controller: textEditingController
  );
  ...
  
  textEditingController.text = 'my text'; // This is where I can set the text in the TextFormField
  </code></pre>
  
  <p>Is there a similar way to programmatically set the selection in a <code>DropdownButton</code>? As far as I know, simply setting the <code>value</code> field in a <code>DropdownButton</code> won't suffice since the change won't be applied without calling the <code>setState</code> from the wrapping <code>state</code> object.</p>