Add TODO comment for legacy inspector (#9738)

packages/devtools_app/lib/src/screens/inspector_shared/inspector_screen_controller.dart

diff --git a/packages/devtools_app/lib/src/screens/inspector_shared/inspector_screen_controller.dart b/packages/devtools_app/lib/src/screens/inspector_shared/inspector_screen_controller.dart
index a009c45..5ca8073 100644
--- a/packages/devtools_app/lib/src/screens/inspector_shared/inspector_screen_controller.dart
+++ b/packages/devtools_app/lib/src/screens/inspector_shared/inspector_screen_controller.dart
@@ -43,6 +43,7 @@
       treeType: FlutterTreeType.widget,
     );
 
+    // TODO(elliette): Remove legacy inspector.
     legacyInspectorTreeController = legacy.InspectorTreeController(
       gaId: InspectorScreenMetrics.summaryTreeGaId,
     );