create new Dart specific completion contributor extension point

pkg/analysis_server/lib/src/services/completion/completion_manager.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 services.completion.manager;
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/analysis_server.dart';
+import 'package:analysis_server/src/plugin/server_plugin.dart';
 import 'package:analysis_server/src/provisional/completion/completion_core.dart'
     show CompletionRequest, CompletionResult;
 import 'package:analysis_server/src/services/completion/dart_completion_manager.dart';
 import 'package:analysis_server/src/services/search/search_engine.dart';
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 
 /**
  * [CompletionCache] contains information about the prior code completion
@@ skipping 210 matching lines @@
   @override
   final Source source;
 
   @override
   final int offset;
 
   CompletionRequestImpl(this.server, this.context, this.source, this.offset);
 
   @override
   ResourceProvider get resourceProvider => server.resourceProvider;
+
+  @override
+  ServerPlugin get serverPlugin => server.serverPlugin;
 }
 
 /**
  * Code completion result generated by an [CompletionManager].
  */
 class CompletionResultImpl implements CompletionResult {
   /**
    * The length of the text to be replaced if the remainder of the identifier
    * containing the cursor is to be replaced when the suggestion is applied
    * (that is, the number of characters in the existing identifier).
@@ skipping 46 matching lines @@
    */
   final String name;
 
   /**
    * The elapse time or `null` if undefined.
    */
   final Duration elapsed;
 
   OperationPerformance(this.name, this.elapsed);
 }