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: class ObjectFieldContext extends Node {
    final Token NAME;
    final Token COLON;
    final ValueContext value;
  
    ObjectFieldContext(this.NAME, this.COLON, this.value);
  
  
negative.code: import 'package:source_span/source_span.dart';
  import '../token.dart';
  import 'input_value.dart';
  import 'node.dart';
  
  class NullValueContext extends InputValueContext<Null> {
    final Token NULL;
  
    NullValueContext(this.NULL);
  