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

pkg/http/test/mock_client_test.dart

Index: pkg/http/test/mock_client_test.dart
diff --git a/pkg/http/test/mock_client_test.dart b/pkg/http/test/mock_client_test.dart
index 7bc39566ff7e780907c0ae8ffb507d8ed68e956d..44d51614f924d658928bf5b975630e134a9221b8 100644
--- a/pkg/http/test/mock_client_test.dart
+++ b/pkg/http/test/mock_client_test.dart
@@ -20,7 +20,7 @@ void main() {
     var client = new MockClient((request) {
       return new Future.immediate(new http.Response(
           JSON.stringify(request.bodyFields), 200,
-          headers: {'content-type': 'application/json'}));
+          request: request, headers: {'content-type': 'application/json'}));
     });
 
     expect(client.post("http://example.com/foo", fields: {