Generate constants in server as we do in plugin

pkg/analysis_server/test/integration/edit/list_postfix_completion_templates_test.dart

 // Copyright (c) 2017, 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.
 
 import 'package:analysis_server/protocol/protocol_generated.dart';
-import 'package:analyzer_plugin/protocol/protocol_common.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import '../support/integration_tests.dart';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(ListPostfixCompletionTemplatesTest);
   });
 }
@@ skipping 16 matching lines @@
     await analysisFinished;
 
     // expect a postfix template list result
     EditListPostfixCompletionTemplatesResult result =
         await sendEditListPostfixCompletionTemplates();
     expect(result.templates, isNotNull);
     expect(result.templates.length, greaterThan(15));
     expect(result.templates[0].runtimeType, PostfixTemplateDescriptor);
   }
 }