Add File.modificationStamp accessor.

pkg/analyzer/test/file_system/physical_resource_provider_test.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 test.physical_file_system;
 
 import 'dart:async';
 import 'dart:io' as io;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/physical_file_system.dart';
 import 'package:analyzer/src/generated/source_io.dart';
 import 'package:path/path.dart';
 import 'package:unittest/unittest.dart';
 import 'package:watcher/watcher.dart';
 
 
 var _isFile = new isInstanceOf<File>();
 var _isFolder = new isInstanceOf<Folder>();
+var _isFileSystemException = new isInstanceOf<FileSystemException>();
 
 
 main() {
   groupSep = ' | ';
 
   group('PhysicalResourceProvider', () {
     io.Directory tempDirectory;
     String tempPath;
 
     setUp(() {
@@ skipping 162 matching lines @@
         File file2 = PhysicalResourceProvider.INSTANCE.getResource(path2);
         expect(file == file2, isFalse);
       });
 
       test('isOrContains', () {
         File file = PhysicalResourceProvider.INSTANCE.getResource(path);
         expect(file.isOrContains(path), isTrue);
         expect(file.isOrContains('foo'), isFalse);
       });
 
+      group('modificationStamp', () {
+        test('exists', () {
+          new io.File(path).writeAsStringSync('contents');
+          File file = PhysicalResourceProvider.INSTANCE.getResource(path);
+          expect(file.modificationStamp, isNonNegative);
+        });
+
+        test('does not exist', () {
+          File file = PhysicalResourceProvider.INSTANCE.getResource(path);
+          expect(() {
+            file.modificationStamp;
+          }, throwsA(_isFileSystemException));
+        });
+      });
+
       test('shortName', () {
         expect(file.shortName, 'file.txt');
       });
 
       test('toString', () {
         expect(file.toString(), path);
       });
 
       test('parent', () {
         Resource parent = file.parent;
@@ skipping 120 matching lines @@
         expect(
             folder.canonicalizePath(join('.', 'baz')),
             equals(join(path, 'baz')));
         expect(
             folder.canonicalizePath(join(path2, '.', 'baz')),
             equals(join(path2, 'baz')));
       });
     });
   });
 }