validate method
Throws a FormatException if this selector uses any variables that don't
appear either in validVariables
or in the set of variables that are
known to be valid for all selectors.
Implementation
void validate(Set<String> validVariables) {
if (identical(this, all)) return;
_wrapFormatException(
() => _inner.validate((name) =>
_universalValidVariables.contains(name) ||
validVariables.contains(name)),
_span);
}