anchor.diff:  
     @override
     Iterable<SyntacticEntity> get childEntities => super._childEntities
  +    ..add(keyword)
       ..add(_uri)
       ..add(deferredKeyword)
       ..add(asKeyword)
  
positive.code: 
    @override
    Iterable<SyntacticEntity> get childEntities => super._childEntities
      ..add(_uri)
      ..addAll(combinators)
      ..add(semicolon);
  
negative.code:   @override
    CompilationUnitElement declaredElement;
  
    Version languageVersion;
  
    /// The line information for this compilation unit.
  