title: How to change the DropdownButton's icon color? Flutter
body: <p>In Flutter, I am trying to change the color of the DropdownButton's icon (the down arrow icon) to white color.</p>
  
  <p>I tried using the style property with no help. The text color became white but the icon is still the default grey.</p>
  
  <pre><code>DropdownButton(
           style: TextStyle(color: Colors.white, decorationColor: 
               Colors.white),
           items: this.items,
           value: null,
           hint: Text(SaveOptions[_saveOption], style: TextStyle(color: 
               Colors.white)),
           onChanged: (selectedOption) {
             setState(() {
               _saveOption = selectedOption;
             });
           })
  </code></pre>
  
  <p>How do I change the color of the arrow icon to white?</p>