Add support for cross-session storage

pkg/analyzer/lib/file_system/file_system.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 file_system;
 
 import 'dart:async';
 
 import 'package:analyzer/src/generated/source.dart';
 import 'package:path/path.dart';
@@ skipping 91 matching lines @@
 abstract class ResourceProvider {
   /**
    * Get the path context used by this resource provider.
    */
   Context get pathContext;
 
   /**
    * Return the [Resource] that corresponds to the given [path].
    */
   Resource getResource(String path);
+
+  /**
+   * Return the folder in which the plugin with the given [pluginId] can store
+   * state that will persist across sessions. The folder returned for a given id
+   * will not be returned for a different id, ensuring that plugins do not need
+   * to be concerned with file name collisions with other plugins, assuming that
+   * the plugin ids are unique. The plugin ids must be valid folder names.
+   */
+  Folder getStateLocation(String pluginId);
 }
 
 
 /**
  * A [UriResolver] for [Resource]s.
  */
 class ResourceUriResolver extends UriResolver {
   /**
    * The name of the `file` scheme.
    */
@@ skipping 17 matching lines @@
   }
 
   /**
    * Return `true` if the given URI is a `file` URI.
    *
    * @param uri the URI being tested
    * @return `true` if the given URI is a `file` URI
    */
   static bool _isFileUri(Uri uri) => uri.scheme == _FILE_SCHEME;
 }