[flutter_tools] add todo for userMessages (#125156)

Adds `todo` link in code for #125155.

packages/flutter_tools/lib/src/base/user_messages.dart

@@ -5,6 +5,12 @@
 import 'context.dart';
 import 'platform.dart';
 
+/// Contains messages produced by Flutter tools.
+//
+// This allows partial reimplementations of the flutter tool to override
+// certain messages.
+// TODO(andrewkolos): It is unclear if this is worth keeping. See
+// https://github.com/flutter/flutter/issues/125155.
 UserMessages get userMessages => context.get<UserMessages>()!;
 
 /// Class containing message strings that can be produced by Flutter tools.