title: ExpansionTile Title color changes on expansion
body: <pre><code>ExpansionTile(
                  title: Text('test'),
                  children: [Text('test expanded')],
                ),
  </code></pre>
  <p>In this simple test, when I click on the title to expand the widget, the original title text turns from black to grey. <p>I'm not sure if this is a Theme issue (I tried changing every grey color to something bright) or is something that I can manage through <code>ExpansionTile</code> (don't see it in the hover menu).</p>
  <p>I did find that I could explicitly state the text color should be black and then it stays constant. However, in my real world use case for this tile I have several Text widgets and don't want to have to color each one.</p>
  <p>Any ideas on how to address this in one shot?</p>
  <p><a href="https://i.stack.imgur.com/TKluz.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/TKluz.png" alt="expanded text in grey" /></a></p>