Use checked-in .package file for building and testing

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.
 
 import 'dart:io';
 
 import "package:async_helper/async_helper.dart";
 
 // NOTE: This test tries to ensure that an HttpClient will close it's
 // underlying idle connections after [HttpClient.idleTimeout].
 //
 // The main script spawns a server and a subprocess which does a connection back
 // to it.
-// The subprocess is expected to shut down it's idle sockets after
+// The subprocess is expected to shut down its idle sockets after
 // [HttpClient.idleTimeout] and the main script will assert that this happens
 // within +/- 2 <= seconds.
 
 const SECONDS = 4;
 
+List<String> packageOptions() {
+  if (Platform.packageRoot != null) {
+    return <String>['--package-root=${Platform.packageRoot}'];
+  } else if (Platform.packageConfig != null) {
+    return <String>['--packages=${Platform.packageConfig}'];
+  } else {
+    return <String>[];
+  }
+}
+
 void runServerProcess() {
   asyncStart();
   HttpServer.bind('127.0.0.1', 0).then((server) {
     var url = 'http://127.0.0.1:${server.port}/';
 
     server.idleTimeout = const Duration(hours: 1);
 
     var subscription = server.listen((HttpRequest request) {
       return request.response..write('hello world')..close();
     });
 
     var sw = new Stopwatch()..start();
-    var arguments = ['--package-root=${Platform.packageRoot}',
-                     '${Platform.script}',
-                     url];
+    var arguments = packageOptions()..add(Platform.script.toString())..add(url);
     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.
@@ skipping 21 matching lines @@
       .then((_) => print('drained client request'));
 }
 
 void main(List<String> args) {
   if (args.length == 1) {
     runClientProcess(args.first);
   } else {
     runServerProcess();
   }
 }