anchor.diff:  
     var selectorGroup = parseSelectorGroupFrom(declaration);
     if (selectorGroup == null) {
  -    logger.warning(declaration.expression.span.message('Not a valid selector'));
  +    logger.severe(declaration.expression.span.message('Not a valid selector'));
       return null;
     }
   
  
positive.code:   var styleSheet = parse(css, errors: errors);
  
    if (errors.isNotEmpty) {
      logger.warning('Errors parsing CSS:\n${errors.join('\n')}');
    }
  
    var shadowTransformer = useLegacyEncapsulation
  
negative.code: 
      if (x.runtimeType == y.runtimeType) {
        if (x is ElementSelector || x is NamespaceSelector) {
          logger.severe('Compound selector contains multiple type selectors:\n'
              '${x.span.message('')}\n'
              '${y.span.message('')}');
        } else if (x is PseudoElementSelector) {
          logger.severe(
              'Compound selector contains multiple pseudo element selectors:\n'
              '${x.span.message('')}\n'
              '${y.span.message('')}');
  