anchor.diff:  
   /// A [BuiltValueToStringHelper] that produces single line output.
   class FlatBuiltValueToStringHelper implements BuiltValueToStringHelper {
  -  StringBuffer _result = StringBuffer();
  +  StringBuffer? _result = StringBuffer();
     bool _previousField = false;
   
     FlatBuiltValueToStringHelper(String className) {
  -    _result..write(className)..write(' {');
  -  }
  -
  -  @override
  -  void add(String field, Object value) {
  +    _result!..write(className)..write(' {');
  +  }
  +
  +  @override
  +  void add(String field, Object? value) {
       if (value != null) {
  -      if (_previousField) _result.write(',');
  -      _result..write(field)..write('=')..write(value);
  +      if (_previousField) _result!.write(',');
  +      _result!..write(field)..write('=')..write(value);
         _previousField = true;
       }
     }
   
     @override
     String toString() {
  -    _result..write('}');
  +    _result!..write('}');
       var stringResult = _result.toString();
       _result = null;
       return stringResult;
  
positive.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('=')
  
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.
  