title: A new way to get showSnackBar
body: <p>According to the updated flutter the <code>showSnackBar</code> on <code>currentState</code> is deprecated.</p>
  <pre><code>void _showSnackBar(String message) {
      _scaffoldKey.currentState.showSnackBar( // it's deprecated
        SnackBar(
          content: Text(message),
        ),
      );
    }
  </code></pre>
  <p>Instead, it proposes to use <code>ScaffoldMessenger.showSnackBar();</code>. But if I use it then the error appears</p>
  <blockquote>
  <p>The method 'showSnackBar' isn't defined for the type
  'ScaffoldMessenger'.</p>
  </blockquote>
  <p>So, how to use <code>showSnackBar</code> now?</p>