rework ArgListContributor to use new task model

pkg/analysis_server/lib/src/provisional/completion/dart/completion_plugin.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.
 
 library analysis_server.src.provisional.completion.dart.plugin;
 
 import 'package:analysis_server/src/provisional/completion/completion.dart';
 import 'package:analysis_server/src/provisional/completion/dart/completion.dart';
 import 'package:analysis_server/src/provisional/completion/dart/completion_dart.dart';
+import 'package:analysis_server/src/services/completion/dart/arglist_contributor.dart';
 import 'package:analysis_server/src/services/completion/dart/completion_manager.dart';
 import 'package:analysis_server/src/services/completion/dart/keyword_contributor.dart';
 import 'package:plugin/plugin.dart';
 
 /**
  * The shared dart completion plugin instance.
  */
 final DartCompletionPlugin dartCompletionPlugin = new DartCompletionPlugin();
 
 class DartCompletionPlugin implements Plugin {
@@ skipping 38 matching lines @@
     //
     // Register DartCompletionManager as a CompletionContributor
     // which delegates to all the DartCompletionContributors
     //
     registerExtension(COMPLETION_CONTRIBUTOR_EXTENSION_POINT_ID,
         () => new DartCompletionManager());
     //
     // Register the default DartCompletionContributors
     //
     registerExtension(DART_COMPLETION_CONTRIBUTOR_EXTENSION_POINT_ID,
+        () => new ArgListContributor());
+    registerExtension(DART_COMPLETION_CONTRIBUTOR_EXTENSION_POINT_ID,
         () => new KeywordContributor());
   }
 
   /**
    * Validate the given extension by throwing an [ExtensionError] if it is not a
    * valid Dart specific completion contributor.
    */
   void _validateDartCompletionContributorExtension(Object extension) {
     if (extension is! DartCompletionContributorFactory) {
       String id = _contributorExtensionPoint.uniqueIdentifier;
       throw new ExtensionError(
           'Extensions to $id must be a DartCompletionContributorFactory');
     }
   }
 }