title: Flutter undefined name WhitelistingTextInputFormatter
body: <p>I'm trying to use <code>WhitelistingTextInputFormatter</code> in a <code>TextFormField</code> in a Flutter app but I get</p>
  
  <blockquote>
    <p>undefined name WhitelistingTextInputFormatter</p>
  </blockquote>
  
  <p>Here is my code:</p>
  
  <pre><code>child: TextFormField(
           keyboardType: TextInputType.number,
           inputFormatters: [WhitelistingTextInputFormatter.digitsOnly],
           decoration: InputDecoration(labelText: 'Staff Number', hintText: 'enter staff number'),
  ),
  </code></pre>