code: void findScopes(Node externs, Node root) {
      NodeTraversal.traverseRoots(
          compiler,
          new NodeTraversal.AbstractScopedCallback() {
            @Override
            public void enterScope(NodeTraversal t) {
              createScopeFrom(t.getScope());
            }
  
            @Override
            public void visit(NodeTraversal t, Node n, Node p) {}
          },
          externs,
          root);
    }
docs: Finds all the scopes and adds them to this symbol table.