anchor.diff:        TypeLookup lookupType,
         DiagnosticReporter reporter,
         CommonElements commonElements,
  -      {List<String> validTags}) {
  +      {List<String>? validTags}) {
       void setSideEffects(SideEffects newEffects) {
         behavior.sideEffects.setTo(newEffects);
       }
  
positive.code:       });
      }
  
      String creates = values['creates'];
      if (creates != null) {
        resolveTypesString(dartTypes, creates, onVoid: () {
          reportError("Invalid type string 'creates:$creates'");
  
negative.code: 
      List<Object> typesReturned = readTypes();
      List<Object> typesInstantiated = readTypes();
      String? codeTemplateText = source.readStringOrNull();
      SideEffects sideEffects = SideEffects.readFromDataSource(source);
      int throwBehavior = source.readInt();
      bool isAllocation = source.readBool();
  