Remove uses of the old plugin model from the analysis server

pkg/analysis_server/lib/plugin/edit/assist/assist.dart

-// Copyright (c) 2015, 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.
-
-/**
- * Support for client code that extends the analysis server by adding new assist
- * contributors.
- *
- * Plugins can register assist contributors. The registered contributors will be
- * used to get assists any time a client issues an 'edit.getAssists' request.
- *
- * If a plugin wants to add assists, it should implement the class
- * [AssistContributor] and then register the contributor by including code like
- * the following in the plugin's registerExtensions method:
- *
- *     @override
- *     void registerExtensions(RegisterExtension registerExtension) {
- *       ...
- *       registerExtension(
- *           ASSIST_CONTRIBUTOR_EXTENSION_POINT_ID,
- *           new MyAssistContributor());
- *       ...
- *     }
- */
-import 'package:analysis_server/plugin/edit/assist/assist_core.dart';
-import 'package:analysis_server/src/plugin/server_plugin.dart';
-import 'package:plugin/plugin.dart';
-
-/**
- * The identifier of the extension point that allows plugins to register assist
- * contributors. The object used as an extension must be an [AssistContributor].
- */
-final String ASSIST_CONTRIBUTOR_EXTENSION_POINT_ID = Plugin.join(
-    ServerPlugin.UNIQUE_IDENTIFIER,
-    ServerPlugin.ASSIST_CONTRIBUTOR_EXTENSION_POINT);