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: class BuiltValueEnum {
    /// The wire name when the enum is serialized. Defaults to `null` which
    /// indicates that the name is to be taken from the literal class name.
    final String wireName;
  
    const BuiltValueEnum({this.wireName});
  }
  
negative.code: 
  /// A [BuiltValueToStringHelper] that produces multi-line indented output.
  class IndentingBuiltValueToStringHelper implements BuiltValueToStringHelper {
    StringBuffer? _result = StringBuffer();
  
    IndentingBuiltValueToStringHelper(String className) {
      _result!..write(className)..write(' {\n');
      _indentingBuiltValueToStringHelperIndent += 2;
    }
  
    @override
    void add(String field, Object? value) {
      if (value != null) {
        _result!
          ..write(' ' * _indentingBuiltValueToStringHelperIndent)
          ..write(field)
          ..write('=')
  