Fix a race condition in pub serve's exitOnClose test.

sdk/lib/_internal/pub/test/serve/utils.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS d.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 pub_tests;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:http/http.dart' as http;
 import 'package:scheduled_test/scheduled_process.dart';
 import 'package:scheduled_test/scheduled_stream.dart';
 import 'package:scheduled_test/scheduled_test.dart';
+import 'package:stack_trace/stack_trace.dart';
 
 import '../../lib/src/utils.dart';
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 
 /// The pub process running "pub serve".
 ScheduledProcess _pubServer;
 
 /// The ephemeral port assign to the running admin server.
 int _adminPort;
@@ skipping 411 matching lines @@
 Future<Map> _jsonRpcRequest(String method, [Map params]) {
   var id = _rpcId++;
   var message = {
     "jsonrpc": "2.0",
     "method": method,
     "id": id
   };
   if (params != null) message["params"] = params;
   _webSocket.add(JSON.encode(message));
 
-  return _webSocketBroadcastStream
-      .firstWhere((response) => response["id"] == id).then((value) {
+  return Chain.track(_webSocketBroadcastStream
+      .firstWhere((response) => response["id"] == id)).then((value) {
     currentSchedule.addDebugInfo(
         "Web Socket request $method with params $params\n"
         "Result: $value");
 
     expect(value["id"], equals(id));
     return value;
   });
 }
 
 /// Returns a [Future] that completes to a URL string for the server serving
@@ skipping 19 matching lines @@
 /// included. Unlike [getServerUrl], this should only be called after the ports
 /// are known.
 String _getServerUrlSync([String root, String path]) {
   if (root == null) root = 'web';
   expect(_ports, contains(root));
   var url = "http://127.0.0.1:${_ports[root]}";
   if (path != null) url = "$url/$path";
   return url;
 }