Reduce flakiness of http_client_stays_alive_test.

tests/standalone/io/http_client_stays_alive_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.
 
 // OtherResources=http_client_stays_alive_test.dart
 
 import 'dart:io';
 
 import "package:async_helper/async_helper.dart";
 
@@ skipping 36 matching lines @@
     Process.run(Platform.executable, arguments).then((res) {
       subscription.cancel();
       if (res.exitCode != 0) {
         throw "Child exited with ${res.exitCode} instead of 0. "
               "(stdout: ${res.stdout}, stderr: ${res.stderr})";
       }
       var seconds = sw.elapsed.inSeconds;
       // NOTE: There is a slight chance this will cause flakiness, but there is
       // no other good way of testing correctness of timing-dependent code
       // form the outside.
-      if (seconds < SECONDS || (SECONDS + 10) < seconds) {
+      if (seconds < SECONDS || (SECONDS + 30) < seconds) {
         throw "Child did exit within $seconds seconds, but expected it to take "
               "roughly $SECONDS seconds.";
       }
 
       asyncEnd();
     });
   });
 }
 
 void runClientProcess(String url) {
@@ skipping 10 matching lines @@
       .then((_) => print('drained client request'));
 }
 
 void main(List<String> args) {
   if (args.length == 1) {
     runClientProcess(args.first);
   } else {
     runServerProcess();
   }
 }