code: public Boolean isPossibleHeaderVar() {
      if (defn == null) {
        throw new NullPointerException(getSourceLocation().toString());
      }
      return defn.kind() == VarDefn.Kind.PARAM
          || defn.kind() == VarDefn.Kind.STATE
          || defn.kind() == VarDefn.Kind.UNDECLARED;
    }
docs: Returns whether this might be header variable reference. A header variable is declared in Soy
  with the @param or @state annotation.