SubMessage.from constructor
Creates the sub-message, given a list of clauses
in the sort of form
that we're likely to get them from parsing a translation file format,
as a list of key, value
where value may in turn be a list.
Implementation
SubMessage.from(this.mainArgument, List clauses, parent) : super(parent) {
for (var clause in clauses) {
this[clause.first] = (clause.last is List) ? clause.last : [clause.last];
}
}