Add File.modificationStamp accessor.

pkg/analyzer/lib/file_system/physical_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 physical_file_system;
 
 import 'dart:async';
 import 'dart:io' as io;
 
 import 'package:analyzer/src/generated/java_io.dart';
@@ skipping 60 matching lines @@
 }
 
 
 /**
  * A `dart:io` based implementation of [File].
  */
 class _PhysicalFile extends _PhysicalResource implements File {
   _PhysicalFile(io.File file) : super(file);
 
   @override
+  int get modificationStamp {
+    try {
+      io.File file = _entry as io.File;
+      return file.lastModifiedSync().millisecondsSinceEpoch;
+    } on io.FileSystemException catch (exception) {
+      throw new FileSystemException(path, exception.message);
+    }
+  }
+
+  @override
   Source createSource([Uri uri]) {
     io.File file = _entry as io.File;
     JavaFile javaFile = new JavaFile(file.absolute.path);
     if (uri == null) {
       uri = javaFile.toURI();
     }
     return new FileBasedSource.con2(uri, javaFile);
   }
 
   @override
@@ skipping 88 matching lines @@
   bool operator ==(other) {
     if (runtimeType != other.runtimeType) {
       return false;
     }
     return path == other.path;
   }
 
   @override
   String toString() => path;
 }