YamlWarningCallback typedef
YamlWarningCallback =
void Function(String message, [SourceSpan? span])
A callback for emitting a warning.
message is the text of the warning. If span is passed, it's the portion
of the document that the warning is associated with and should be included
in the printed warning.
Implementation
typedef YamlWarningCallback = void Function(String message, [SourceSpan? span]);