anchor.diff:  
   class Symbol {}
   
  +class Set {}
  +
   class Type {}
   
   class _InvocationMirror {
  
positive.code:   cancel() {}
  }
  """;
  
negative.code: 
        case "dart:async":
          return utf8.encode(defaultDartAsyncSource);
  
        case "dart:collection":
          return utf8.encode(defaultDartCollectionSource);
  
        case "dart:_internal":
          return utf8.encode(defaultDartInternalSource);
  
        default:
          return utf8.encode(message == null ? "" : "/* ${message.message} */");
  