Fix async/await type checking in analyzer.

pkg/analysis_server/test/analysis_server_test.dart

 // Copyright (c) 2014, 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.analysis_server;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/constants.dart';
@@ skipping 253 matching lines @@
           // It's normal for this notification to be sent twice.
           // TODO(paulberry): why?
           break;
         default:
           if (!notificationTypesReceived.add(notificationType)) {
             fail('Notification type $notificationType received more than once');
           }
           break;
       }
     }
-    return null; // Work around dartbug.com/22091
   }
 
   Future test_prioritySourcesChangedEvent() {
     resourceProvider.newFolder('/foo');
 
     int eventCount = 0;
     Source firstSource = null;
     server.onPriorityChange.listen((PriorityChangeEvent event) {
       ++eventCount;
       firstSource = event.firstSource;
@@ skipping 97 matching lines @@
   @override
   Response handleRequest(Request request) {
     if (request.method == 'echo') {
       return new Response(request.id, result: {
         'echo': true
       });
     }
     return null;
   }
 }