Remove FileSystem.lastModified

pkg/front_end/lib/physical_file_system.dart

Index: pkg/front_end/lib/physical_file_system.dart
diff --git a/pkg/front_end/lib/physical_file_system.dart b/pkg/front_end/lib/physical_file_system.dart
index 59402a322e01b50b2bcae0e565046a482b09c163..2fe685619524c6ebea049d110332ebef42263932 100644
--- a/pkg/front_end/lib/physical_file_system.dart
+++ b/pkg/front_end/lib/physical_file_system.dart
@@ -51,15 +51,6 @@ class _PhysicalFileSystemEntity implements FileSystemEntity {
     }
   }
 
-  @override
-  Future<DateTime> lastModified() async {
-    try {
-      return await new io.File.fromUri(uri).lastModified();
-    } on io.FileSystemException catch (exception) {
-      throw _toFileSystemException(exception);
-    }
-  }
-
   @override
   Future<List<int>> readAsBytes() async {
     try {