Detect an old PhantomJS and emit a nice error.

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 311 matching lines @@
   }
 
   /// Returns the [BrowserManager] for [platform], which should be a browser.
   ///
   /// If no browser manager is running yet, starts one.
   Future<BrowserManager> _browserManagerFor(TestPlatform platform) {
     var manager = _browserManagers[platform];
     if (manager != null) return manager;
 
     var completer = new Completer();
-    _browserManagers[platform] = completer.future;
+
+    // Swallow errors, since they're already being surfaced through the return
+    // value and [browser.onError].
+    _browserManagers[platform] = completer.future.catchError((_) {});
     var path = _webSocketHandler.create(webSocketHandler((webSocket) {
       completer.complete(new BrowserManager(webSocket));
     }));
 
     var webSocketUrl = url.replace(scheme: 'ws').resolve(path);
 
     var hostUrl = (_pubServeUrl == null ? url : _pubServeUrl)
         .resolve('packages/test/src/runner/browser/static/index.html');
 
     var browser = _newBrowser(hostUrl.replace(queryParameters: {
@@ skipping 45 matching lines @@
       if (_pubServeUrl == null) {
         new Directory(_compiledDir).deleteSync(recursive: true);
       } else {
         _http.close();
       }
 
       _closeCompleter.complete();
     }).catchError(_closeCompleter.completeError);
   }
 }