Switch analysis_server to use 'package:test'.

pkg/analysis_server/test/operation/operation_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.operation;
 
 import 'package:analysis_server/src/operation/operation.dart';
-import 'package:unittest/unittest.dart';
+import 'package:test/test.dart';
 
 import '../utils.dart';
 
 main() {
   initializeTestEnvironment();
 
   group('ServerOperationPriority', () {
     test('toString', () {
       expect(ServerOperationPriority.ANALYSIS.toString(), 'ANALYSIS');
     });
   });
 }