move dart completion contributor instantiation

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/dart/completion.dart';
 import 'package:analysis_server/src/provisional/completion/dart/completion_dart.dart';
 import 'package:analysis_server/src/services/completion/dart/keyword_contributor.dart';
 import 'package:plugin/plugin.dart';
@@ skipping 20 matching lines @@
   /**
    * The extension point that allows plugins to register Dart specific
    * completion contributor factories.
    */
   ExtensionPoint _contributorExtensionPoint;
 
   @override
   String get uniqueIdentifier => UNIQUE_IDENTIFIER;
 
   /**
-   * Return a list containing all of the Dart specific completion contributor
-   * factories that were contributed.
+   * Return a list containing all of the Dart specific completion contributors.
    */
-  List<DartCompletionContributorFactory> get contributorFactories =>
-      _contributorExtensionPoint.extensions;
+  Iterable<DartCompletionContributorFactory> get contributors =>
+      _contributorExtensionPoint.extensions
+          .map((DartCompletionContributorFactory factory) => factory());
 
   @override
   void registerExtensionPoints(RegisterExtensionPoint registerExtensionPoint) {
     _contributorExtensionPoint = registerExtensionPoint(
         CONTRIBUTOR_EXTENSION_POINT,
         _validateDartCompletionContributorExtension);
   }
 
   @override
   void registerExtensions(RegisterExtension registerExtension) {
     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');
     }
   }
 }