anchor.diff:    FileSpan get span => NULL.span;
   
     @override
  -  Null get value => null;
  +  Null computeValue(Map<String, dynamic> variables) => null;
   }
   
  -class EnumValueContext extends ValueContext<String> {
  +class EnumValueContext extends InputValueContext<String> {
     final Token NAME;
   
     EnumValueContext(this.NAME);
  
positive.code: import 'package:source_span/source_span.dart';
  
  import '../token.dart';
  import 'node.dart';
  import 'value.dart';
  
  class NullValueContext extends ValueContext<Null> {
    final Token NULL;
  
    NullValueContext(this.NULL);
  
negative.code:   }
  
    @override
    Map<String, dynamic> computeValue(Map<String, dynamic> variables) {
      if (fields.isEmpty) {
        return <String, dynamic>{};
      } else {
        return fields.fold<Map<String, dynamic>>(<String, dynamic>{},
            (map, field) {
          return map..[field.NAME.text] = field.value.computeValue(variables);
        });
      }
    }
  