Remove uses of the old plugin model from the analysis server

pkg/analysis_server/test/edit/postfix_completion_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.dart';
 import 'package:analysis_server/protocol/protocol_generated.dart';
 import 'package:analysis_server/src/edit/edit_domain.dart';
 import 'package:analyzer_plugin/protocol/protocol_common.dart';
-import 'package:plugin/manager.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import '../analysis_abstract.dart';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(PostfixCompletionTest);
   });
 }
 
 @reflectiveTest
 class PostfixCompletionTest extends AbstractAnalysisTest {
   SourceChange change;
 
   @override
   void setUp() {
     super.setUp();
     createProject();
-    ExtensionManager manager = new ExtensionManager();
-    manager.processPlugins([server.serverPlugin]);
     handler = new EditDomainHandler(server);
   }
 
   test_for() async {
     addTestFile('''
 main() {
   [].for
 }
 ''');
     await waitForTasksFinished();
@@ skipping 46 matching lines @@
     if (!isApplicable.value) {
       fail("Postfix completion not applicable at given location");
     }
     request = new EditGetPostfixCompletionParams(testFile, key, offset)
         .toRequest('1');
     response = await waitResponse(request);
     var result = new EditGetPostfixCompletionResult.fromResponse(response);
     change = result.change;
   }
 }