After #5270 (I think this might be the cause), if you make a code like this (no actual file with this name):

export 'MyFile.dart';

You'll see the new message for it. But even if you replace this export completely (no second file needed) the message will persist.

The file name 'MyFile.dart' isn't a lower_case_with_underscores identifier.
Try changing the name to follow the lower_case_with_underscores style.
You may need to close the file and restart VS Code after renaming a file by only casing.

This is persisted because the message is showing on a non-existing file.