anchor.diff:  ///                 iconSize: 24,
   ///                 elevation: 16,
   ///                 style: TextStyle(color: Colors.deepPurple),
  -///                 onChanged: (String newValue) {
  +///                 onChanged: (String? newValue) {
   ///                   setState(() {
   ///                     _dropdownValue = newValue;
   ///                   });
  
positive.code: /// // An example of a type that someone might want to autocomplete a list of.
  /// class User {
  ///   const User({
  ///     this.email,
  ///     this.name,
  ///   });
  ///
  ///   final String email;
  
negative.code: ///                 decoration: InputDecoration(
  ///                   hintText: 'This is a regular TextFormField',
  ///                 ),
  ///                 validator: (String? value) {
  ///                   if (value == null || value.isEmpty) {
  ///                     return 'Can\'t be empty.';
  ///                   }
  ///                   return null;
  