Fix uses_dynamic_as_bottom error (#63)

diff --git a/lib/src/backends/memory/memory_file_system_entity.dart b/lib/src/backends/memory/memory_file_system_entity.dart
index dfe23f9..05dd9a1 100644
--- a/lib/src/backends/memory/memory_file_system_entity.dart
+++ b/lib/src/backends/memory/memory_file_system_entity.dart
@@ -209,7 +209,7 @@
   /// [_defaultCheckType] is used to perform this validation.
   FileSystemEntity _renameSync(
     String newPath, {
-    _RenameOverwriteValidator<dynamic> validateOverwriteExistingEntity,
+    _RenameOverwriteValidator<_Node> validateOverwriteExistingEntity,
     bool followTailLink: false,
     _TypeChecker checkType,
   }) {