title: how to make RaisedButton auto fit screen width in flutter
body: <p>Now I am define the RaisedButton width like this in flutter:</p>
  <pre><code>builder: (context) {
       return ButtonTheme(
              minWidth: 400.0,
              height: 50.0,
              child: RaisedButton()
  )}
  </code></pre>
  <p>but it could not fit all type of different width of screen, how to make it auto fit? I have tried using 80%, but it seems not work as percent.</p>