anchor.diff:            'this tag will not include the default properties of "$baseTag". '
             'To fix this, either write this tag as <$baseTag '
             'is="$customTagName"> or remove the "extends" attribute from '
  -          'the custom element declaration.', node.sourceSpan);
  +          'the custom element declaration.', span: node.sourceSpan);
         return;
       }
   
  
positive.code:         if (existing.hasConflict) continue;
          existing.hasConflict = true;
          logger.warning('duplicate definition for custom tag "$name".',
            existing.span);
          logger.warning('duplicate definition for custom tag "$name" '
            ' (second definition).', span);
          continue;
        }
  
  
negative.code:   noSuchMethod(Invocation m) => reflect(_original).delegate(m);
  
    // form TransformLogger:
    void warning(String message, {AssetId asset, Span span})
        => _write('warning', message, span);
  
    void error(String message, {AssetId asset, Span span})
        => _write('error', message, span);
  
    void _write(String kind, String message, Span span) {
      _messages.add(_formatter(kind, message, span));
  