Add File.modificationStamp accessor.

pkg/analyzer/test/file_system/physical_resource_provider_test.dart

Index: pkg/analyzer/test/file_system/physical_resource_provider_test.dart
diff --git a/pkg/analyzer/test/file_system/physical_resource_provider_test.dart b/pkg/analyzer/test/file_system/physical_resource_provider_test.dart
index c1c09ca753ca501453939a62280dd645b0d296d5..3ba46e898290f598ebca2f8fa5191392562f1925 100644
--- a/pkg/analyzer/test/file_system/physical_resource_provider_test.dart
+++ b/pkg/analyzer/test/file_system/physical_resource_provider_test.dart
@@ -17,6 +17,7 @@ import 'package:watcher/watcher.dart';
 
 var _isFile = new isInstanceOf<File>();
 var _isFolder = new isInstanceOf<Folder>();
+var _isFileSystemException = new isInstanceOf<FileSystemException>();
 
 
 main() {
@@ -199,6 +200,21 @@ main() {
         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');
       });