code: public void visitApply(JCMethodInvocation tree) {
              if (TreeInfo.name(tree.meth) == names._super) {
                  addFreeVars((ClassSymbol) TreeInfo.symbol(tree.meth).owner);
              }
              super.visitApply(tree);
          }
docs: If tree refers to a superclass constructor call,
  add all free variables of the superclass.