Add support for cross-session storage

pkg/analyzer/lib/file_system/file_system.dart

Index: pkg/analyzer/lib/file_system/file_system.dart
diff --git a/pkg/analyzer/lib/file_system/file_system.dart b/pkg/analyzer/lib/file_system/file_system.dart
index 5ef0196c56ca176b677724b7ebd42dae76a321fc..c2fb88f6e9dcaa597445a30d7aa2c0fa0d234499 100644
--- a/pkg/analyzer/lib/file_system/file_system.dart
+++ b/pkg/analyzer/lib/file_system/file_system.dart
@@ -109,6 +109,15 @@ abstract class ResourceProvider {
    * 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);
 }