refactor context to be implicit-downcast safe (#31622)

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

diff --git a/packages/flutter_tools/lib/src/base/user_messages.dart b/packages/flutter_tools/lib/src/base/user_messages.dart
index 104d364..58b83e8 100644
--- a/packages/flutter_tools/lib/src/base/user_messages.dart
+++ b/packages/flutter_tools/lib/src/base/user_messages.dart
@@ -4,7 +4,7 @@
 
 import 'context.dart';
 
-UserMessages get userMessages => context[UserMessages];
+UserMessages get userMessages => context.get<UserMessages>();
 
 /// Class containing message strings that can be produced by Flutter tools.
 class UserMessages {