prepare for lint update of prefer_final_fields (#35059)

packages/flutter_tools/lib/src/artifacts.dart

diff --git a/packages/flutter_tools/lib/src/artifacts.dart b/packages/flutter_tools/lib/src/artifacts.dart
index 319840d..1c7c178 100644
--- a/packages/flutter_tools/lib/src/artifacts.dart
+++ b/packages/flutter_tools/lib/src/artifacts.dart
@@ -260,7 +260,7 @@
 
   final String _engineSrcPath;
   final String engineOutPath; // TODO(goderbauer): This should be private.
-  String _hostEngineOutPath;
+  final String _hostEngineOutPath;
 
   @override
   String getArtifactPath(Artifact artifact, { TargetPlatform platform, BuildMode mode }) {