title: Flutter update countertext
body: <p>In Flutter, <code>inputDecoration's</code> countertext property does not change as the user is typing in the <code>TextFormField</code>. Is it possible to decrement the countertext as the user is typing?</p>
  
  <pre><code>TextFormField(
   keyboardType: TextInputType.number,
   decoration: new InputDecoration(
     counterText: "9",
     hintText: "Enter exact order number",
    ),
  )
  </code></pre>