Add File.modificationStamp accessor.

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 ff3e5ff926e277e0932989a785c7f3edda4f9f15..a53f0e6c069466dc6ebd357bc3dedb971fa1c953 100644
--- a/pkg/analyzer/lib/file_system/file_system.dart
+++ b/pkg/analyzer/lib/file_system/file_system.dart
@@ -16,6 +16,12 @@ import 'package:watcher/watcher.dart';
  */
 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]);
@@ -23,6 +29,19 @@ abstract class File extends Resource {
 
 
 /**
+ * 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 {