Add File.modificationStamp accessor.

pkg/analyzer/lib/file_system/physical_file_system.dart

Index: pkg/analyzer/lib/file_system/physical_file_system.dart
diff --git a/pkg/analyzer/lib/file_system/physical_file_system.dart b/pkg/analyzer/lib/file_system/physical_file_system.dart
index 3f3e452a4afcbd73d87927002a9956d1ce972e4f..398e451d63e224e6d1dc6b1aefe501e6e1202a90 100644
--- a/pkg/analyzer/lib/file_system/physical_file_system.dart
+++ b/pkg/analyzer/lib/file_system/physical_file_system.dart
@@ -78,6 +78,16 @@ 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);