Add support for specifying files needing analysis

pkg/analysis_server/lib/plugin/index.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 index
  * contributors.
  */
 library analysis_server.plugin.index;
 
 import 'package:analysis_server/analysis/index/index_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
- * index contributors with the server. The object used as an extension must be
- * an [IndexContributor].
+ * The identifier of the extension point that allows plugins to register index
+ * contributors. The object used as an extension must be an [IndexContributor].
  */
 final String INDEX_CONTRIBUTOR_EXTENSION_POINT_ID = Plugin.join(
     ServerPlugin.UNIQUE_IDENTIFIER,
     ServerPlugin.INDEX_CONTRIBUTOR_EXTENSION_POINT);