anchor.diff:  /// version increase.
   abstract class BuiltValueToStringHelper {
     /// Add a field and its value.
  -  void add(String field, Object value);
  +  void add(String field, Object? value);
   
     /// Returns to completed toString(). The helper may not be used after this
     /// method is called.
  
positive.code:   ///
    /// Or, set [wireNumber] to serialize to an `int`. Only one of the two may be
    /// used.
    final String wireName;
  
    /// The wire name when the constant is serialized. Defaults to `null` which
    /// indicates the name is to be taken from the literal field name.
    ///
    /// Or, set [wireNumber] to serialize to a `String`. Only one of the two may
    /// be used.
    final int wireNumber;
  
    /// Marks a value that is used as a fallback when an unrecognized value
    /// is encountered.
  
negative.code:   @override
    String toString() {
      _indentingBuiltValueToStringHelperIndent -= 2;
      _result!..write(' ' * _indentingBuiltValueToStringHelperIndent)..write('}');
      var stringResult = _result.toString();
      _result = null;
      return stringResult;
  