Switch analysis_server to use 'package:test'.

pkg/analysis_server/test/services/completion/test_all.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.services.completion;
 
-import 'package:unittest/unittest.dart';
+import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import '../../utils.dart';
 import 'completion_target_test.dart' as completion_target_test;
 import 'dart/test_all.dart' as dart_contributor_tests;
 
 /// Utility for manually running all tests.
 main() {
   initializeTestEnvironment();
-  group('completion', () {
+  defineReflectiveSuite(() {
     completion_target_test.main();
     dart_contributor_tests.main();
-  });
+  }, name: 'completion');
 }