Add support for specifying files needing analysis

pkg/analysis_server/lib/plugin/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.
  */
 library analysis_server.plugin.assist;
 
 import 'package:analysis_server/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 new
- * assist contributors with the server. The object used as an extension must be
- * an [AssistContributor].
+ * 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);