code: void fillSymbolVisibility(Node externs, Node root) {
      CollectFileOverviewVisibility collectPass = new CollectFileOverviewVisibility(compiler);
      collectPass.process(externs, root);
      ImmutableMap<StaticSourceFile, Visibility> visibilityMap =
          collectPass.getFileOverviewVisibilityMap();
  
      NodeTraversal.traverseRoots(
          compiler,
          new VisibilityCollector(visibilityMap, compiler.getCodingConvention()),
          externs,
          root);
    }
docs: Records the visibility of each symbol.