pkg/analysis_server: fixes for unittest deprecations

pkg/analysis_server/test/channel_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.channel;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/channel.dart';
 import 'package:analysis_server/src/protocol.dart';
-import 'package:unittest/matcher.dart';
 import 'package:unittest/unittest.dart';
 
 import 'mocks.dart';
 
 main() {
   group('WebSocketChannel', () {
     setUp(WebSocketChannelTest.setUp);
     test('close', WebSocketChannelTest.close);
     test('invalidJsonToClient', WebSocketChannelTest.invalidJsonToClient);
     test('invalidJsonToServer', WebSocketChannelTest.invalidJsonToServer);
@@ skipping 129 matching lines @@
   }
 
   static void expectMsgCount({requestCount: 0,
                               responseCount: 0,
                               notificationCount: 0}) {
     expect(requestsReceived, hasLength(requestCount));
     expect(responsesReceived, hasLength(responseCount));
     expect(notificationsReceived, hasLength(notificationCount));
   }
 }