anchor.diff:    ///
     /// Set to `false` to ignore the field when calculating `hashCode` and when
     /// comparing with `operator==`.
  -  final bool compare;
  +  final bool? compare;
   
     /// Whether the field is serialized. Defaults to `null` which means
     /// [BuiltValue.defaultSerialize] is used.
     ///
     /// If a field is not serialized, it must either be `@nullable` or specify a
     /// default for deserialization to succeed.
  -  final bool serialize;
  +  final bool? serialize;
   
     /// The wire name when the field is serialized. Defaults to `null` which
     /// indicates the name is to be taken from the literal field name.
  -  final String wireName;
  +  final String? wireName;
   
     const BuiltValueField({this.compare, this.serialize, this.wireName});
   }
  
positive.code:   @override
    String toString() {
      _indentingBuiltValueToStringHelperIndent -= 2;
      _result..write(' ' * _indentingBuiltValueToStringHelperIndent)..write('}');
      var stringResult = _result.toString();
      _result = null;
      return stringResult;
  
negative.code: /// version increase.
  abstract class BuiltValueToStringHelper {
    /// Add a field and its value.
    void add(String field, Object? value);
  
    /// Returns to completed toString(). The helper may not be used after this
    /// method is called.
  