anchor.diff:    /// made up of the [proxy] with the sub-path appended. The response from that
     /// proxy request is forwarded back as the response.
     ///
  -  /// A "Via" header is always added to the proxy request. The value of [via]
  -  /// is used, unless it is null or the empty string. Otherwise, a default
  -  /// value is used. A "Via" header containing the same value is added to the
  -  /// response, unless [includeViaHeaderInResponse] is false.
  +  /// A "Via" header is always added to the proxy request, but is optional for
  +  /// the proxy response. The value of [receivedBy] is used to generate the
  +  /// value of the proxy's Via header. A default value is used if _receivedBy_
  +  /// is null or the empty string.
  +  ///
  +  /// The value of [includeViaHeaderInResponse] controls whether the proxy's
  +  /// Via header is added to the proxy response or not. It defaults to true.
   
     Proxy(this.method, String pattern, String proxy,
  -      {String via,
  +      {String receivedBy,
         this.includeViaHeaderInResponse = true,
         @deprecated Iterable<String> requestBlockHeaders,
         @deprecated Iterable<String> responseBlockHeaders})
  -      : _via = (via != null && via.isNotEmpty) ? via : _viaDefault {
  +      : _receivedBy = (receivedBy?.isNotEmpty ?? false)
  +            ? receivedBy
  +            : _receivedByDefault {
       if (method != 'GET' && method != 'HEAD') {
         throw ArgumentError.value(
             method, 'method', 'only GET and HEAD supported');
  
positive.code: 
    String _pathPrefix;
  
    /// Value for the "Via" header.
  
    final String _via;
  
    /// Indicates if a Via header is added to the response.
  
    final bool includeViaHeaderInResponse;
  
  
negative.code:       // proxy response, since this new "Via" header must appear _after_ any
        // previous ones.
  
        final _via = '1.1 $_receivedBy'; // assumes _http.get_ uses HTTP 1.1
        resp.headerAdd('via', _via);
        _logProxyResponse.finest('[${req.id}] + via=$_via');
      }
  