title: How to specify disabledTextColor when migrating from FlatButton to TextButton
body: <p>Consider the following code that specifies <code>disabledTextColor</code> for a <code>FlatButton</code>:</p>
  <pre class="lang-dart prettyprint-override"><code>FlatButton.icon(
    icon: const Icon(Icons.check_box),
    label: const Text('Foo'),
    disabledTextColor: Colors.black,
    onPressed: null,
  ),
  </code></pre>
  <p>How can I translate the <code>disabledTextColor</code> to the equivalent for <code>TextButton</code>?</p>
  <p>I figure I need to override <code>style</code>, but I can't seem to get <a href="https://api.flutter.dev/flutter/material/TextButton/styleFrom.html" rel="noreferrer"><code>TextButton.styleFrom</code></a> to work, and <code>Theme.of(context).textButtonTheme.style</code> is <em>null</em>.</p>