Add an additional closed check to BrowserServer.

lib/src/runner/browser/server.dart

 // Copyright (c) 2015, 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.
 
 library test.runner.browser.server;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
@@ skipping 147 matching lines @@
   Future<Suite> loadSuite(String path) {
     return new Future.sync(() {
       if (_pubServeUrl != null) {
         var suitePrefix = p.withoutExtension(p.relative(path, from: 'test')) +
             '.browser_test';
         var jsUrl = _pubServeUrl.resolve('$suitePrefix.dart.js');
         return _pubServeSuite(path, jsUrl)
             .then((_) => _pubServeUrl.resolve('$suitePrefix.html'));
       } else {
         return _compileSuite(path).then((dir) {
+          if (_closed) return null;
+
           // Add a trailing slash because at least on Chrome, the iframe's
           // window.location.href will do so automatically, and if that differs
           // from the original URL communication will fail.
           return url.resolve(
               "/" + p.toUri(p.relative(dir, from: _compiledDir)).path + "/");
         });
       }
     }).then((suiteUrl) {
       if (_closed) return null;
 
@@ skipping 84 matching lines @@
       if (_pubServeUrl == null) {
         new Directory(_compiledDir).deleteSync(recursive: true);
       } else {
         _http.close();
       }
 
       _closeCompleter.complete();
     }).catchError(_closeCompleter.completeError);
   }
 }