Add File.modificationStamp accessor.

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';
 import 'package:watcher/watcher.dart';
 
 
 /**
  * [File]s are leaf [Resource]s which contain data.
  */
 abstract class File extends Resource {
   /**
+   * Return the last-modified stamp of the file.
+   * Throws [FileSystemException] if the file does not exist.
+   */
+  int get modificationStamp;
+
+  /**
    * Create a new [Source] instance that serves this file.
    */
   Source createSource([Uri uri]);
 }
 
 
 /**
+ * Base class for all file system exceptions.
+ */
+class FileSystemException implements Exception {
+  final String path;
+  final String message;
+
+  FileSystemException(this.path, this.message);
+
+  String toString() => 'FileSystemException(path=$path; message=$message)';
+}
+
+
+/**
  * [Folder]s are [Resource]s which may contain files and/or other folders.
  */
 abstract class Folder extends Resource {
   /**
    * Watch for changes to the files inside this folder (and in any nested
    * folders, including folders reachable via links).
    */
   Stream<WatchEvent> get changes;
 
   /**
@@ skipping 115 matching lines @@
   Uri restoreAbsolute(Source source) => source.uri;
 
   /**
    * 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;
 }