code: void fillSuperReferences(Node externs, Node root) {
      NodeTraversal.Callback collectSuper =
          new AbstractPostOrderCallback() {
            @Override
            public void visit(NodeTraversal t, Node n, Node parent) {
              // Process only 'super' nodes with types.
              if (!n.isSuper() || n.getJSType() == null) {
                return;
              }
              Symbol classSymbol = getSymbolForTypeHelper(n.getJSType(), /* linkToCtor= */ false);
              if (classSymbol != null) {
                classSymbol.defineReferenceAt(n);
              }
            }
          };
      NodeTraversal.traverseRoots(compiler, collectSuper, externs, root);
    }
docs: Fill in references to "super" variables.