Remove FileSystem.lastModified

pkg/front_end/lib/file_system.dart

Index: pkg/front_end/lib/file_system.dart
diff --git a/pkg/front_end/lib/file_system.dart b/pkg/front_end/lib/file_system.dart
index d0b5975017b8f7802d00c4edc2ee869018f1e74d..1306c0c19288cdf5616e3aa617491186e5f4da40 100644
--- a/pkg/front_end/lib/file_system.dart
+++ b/pkg/front_end/lib/file_system.dart
@@ -42,11 +42,6 @@ abstract class FileSystemEntity {
   /// Whether this file system entity exists.
   Future<bool> exists();
 
-  /// Extracts the last-modification time of the file system entity, if it
-  /// exists and it is a file, otherwise the future is completed with
-  /// [FileSystemException].
-  Future<DateTime> lastModified();
-
   /// Attempts to access this file system entity as a file and read its contents
   /// as raw bytes.
   ///