VariableSubstitution.named constructor
Create a substitution based on the name rather than the index. The name may have been used as all upper-case in the translation tool, so we save it separately and look it up case-insensitively once the parent (and its arguments) are definitely available.
Implementation
VariableSubstitution.named(String name, Message? parent) : super(parent) {
_variableNameUpper = name.toUpperCase();
}