Delay HttpClientResponse until HttpClientRequest is fully sent.

tests/standalone/io/regress_6521_test.dart

-// Copyright (c) 2013, 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.
-//
-// Regression test for http://code.google.com/p/dart/issues/detail?id=6393.
-
-import "dart:io";
-import "dart:uri";
-
-var client = new HttpClient();
-var clientRequest;
-
-void main() {
-  HttpServer.bind("127.0.0.1", 0)
-      .then((server) {
-        server.listen(
-            (req) {
-              req.pipe(req.response);
-            });
-
-        client.openUrl("POST", Uri.parse("http://localhost:${server.port}/"))
-            .then((request) {
-              // Keep a reference to the client request object.
-              clientRequest = request;
-              request.writeBytes([0]);
-              return request.response;
-            })
-            .then((response) {
-              // Wait with closing the client request until the response headers
-              // are done.
-              clientRequest.close();
-              response.listen(
-                  (_) {},
-                  onDone: () {
-                    client.close();
-                    server.close();
-                  });
-            });
-      });
-}