More gracefully handle browser errors.

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 248 matching lines @@
           .then((_) {
         if (_closed) return null;
         return url.resolveUri(p.toUri(
             p.withoutExtension(p.relative(path, from: _root)) + ".html"));
       });
     }).then((suiteUrl) {
       if (_closed) return null;
 
       // TODO(nweiz): Don't start the browser until all the suites are compiled.
       return _browserManagerFor(browser).then((browserManager) {
-        if (_closed) return null;
+        if (_closed || browserManager == null) return null;
         return browserManager.loadSuite(path, suiteUrl, metadata);
       }).then((suite) {
         if (_closed) return null;
         if (suite != null) return suite.change(platform: browser.name);
 
         // If the browser manager fails to load a suite and the server isn't
         // closed, it's probably because the browser failed. We emit the failure
         // here to ensure that it gets surfaced.
         return _browsers[browser].onExit;
       });
@@ skipping 133 matching lines @@
       if (_pubServeUrl == null) {
         new Directory(_compiledDir).deleteSync(recursive: true);
       } else {
         _http.close();
       }
 
       _closeCompleter.complete();
     }).catchError(_closeCompleter.completeError);
   }
 }