anchor.diff:      ]);
     }
   
  +  /// Specify hosts internal to Dart. Non-internal packages
  +  /// should not reference internal hosts.
     void addInternalHosts(Iterable<Pattern> hosts) {
       _internalHosts.addAll(hosts);
     }
  
positive.code:     if (!isExternal && !isInternalPackage) {
        return UrlStatus.internal;
      }
      final exists = await safeUrlCheck(uri);
      return exists ? UrlStatus.exists : UrlStatus.missing;
    }
  }
  
  
negative.code:   exists,
  }
  
  /// Checks if an URL is valid and accessible.
  class UrlChecker {
    final _internalHosts = <Pattern>{};
    final _validUrlCache = <String>{};
    final int _maxCacheSize;
  
    UrlChecker({
      int maxCacheSize,
    }) : _maxCacheSize = maxCacheSize ?? 10000 {
      addInternalHosts([
        'dart.dev',
        RegExp(r'.*\.dart\.dev'),
  