Remove FileSystem.lastModified

pkg/analyzer/test/src/summary/resynthesize_kernel_test.dart

Index: pkg/analyzer/test/src/summary/resynthesize_kernel_test.dart
diff --git a/pkg/analyzer/test/src/summary/resynthesize_kernel_test.dart b/pkg/analyzer/test/src/summary/resynthesize_kernel_test.dart
index 12cada7c40caf5f567c87aa8bd0a07b78493d5cb..b3421a59b8470738b9a28e2924e2c54691b45e46 100644
--- a/pkg/analyzer/test/src/summary/resynthesize_kernel_test.dart
+++ b/pkg/analyzer/test/src/summary/resynthesize_kernel_test.dart
@@ -2221,11 +2221,6 @@ class _FileSystemEntityAdaptor implements FileSystemEntity {
     return file.exists;
   }
 
-  @override
-  Future<DateTime> lastModified() async {
-    return new DateTime.fromMicrosecondsSinceEpoch(file.modificationStamp);
-  }
-
   @override
   Future<List<int>> readAsBytes() async {
     return file.readAsBytesSync();