Keep around a copy of the http.Request that triggered each Response.

pkg/http/lib/src/base_response.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library base_response;
 
 import 'dart:io';
 
+import 'base_request.dart';
+
 /// The base class for HTTP responses.
 ///
 /// Subclasses of [BaseResponse] are usually not constructed manually; instead,
 /// they're returned by [BaseClient.send] or other HTTP client methods.
 abstract class BaseResponse {
+  /// The (frozen) request that triggered this response.
+  final BaseRequest request;

[Bob Nystrom, 2012/11/29 23:57:04]

Pedantic, but you should have a test that you can access this property and that
it returns the same object you passed to the constructor.

[nweiz, 2012/11/30 00:08:54]

Done.

+
   /// The status code of the response.
   final int statusCode;
 
   /// The reason phrase associated with the status code.
   final String reasonPhrase;
 
   /// The size of the response body, in bytes. If the size of the request is not
   /// known in advance, this is -1.
   final int contentLength;
 
   // TODO(nweiz): automatically parse cookies from headers
 
   // TODO(nweiz): make this a HttpHeaders object.
   /// The headers for this response.
   final Map<String, String> headers;
 
   /// Whether this response is a redirect.
   final bool isRedirect;
 
   /// Whether the server requested that a persistent connection be maintained.
   final bool persistentConnection;
 
   /// Creates a new HTTP response.
   BaseResponse(
       this.statusCode,
       this.contentLength,
-      {this.headers: const <String>{},
+      {this.request,
+       this.headers: const <String>{},
        this.isRedirect: false,
        this.persistentConnection: true,
        this.reasonPhrase});
 }