code: public void visitApply(JCMethodInvocation tree) {
              if (TreeInfo.name(tree.meth) == names._super) {
                  Symbol constructor = TreeInfo.symbol(tree.meth);
                  ClassSymbol c = (ClassSymbol)constructor.owner;
                  if (c.hasOuterInstance() &&
                      !tree.meth.hasTag(SELECT) &&
                      outerThisStack.head != null)
                      visitSymbol(outerThisStack.head);
              }
              super.visitApply(tree);
          }
docs: If tree refers to a superclass constructor call,
  add all free variables of the superclass.