code: private void ensureArrowFunctionsHaveBlockBodies(NodeTraversal t, Node function) {
      Node body = function.getLastChild();
      if (!body.isBlock()) {
        body.detach();
        Node replacement = IR.block(IR.returnNode(body)).useSourceInfoIfMissingFromForTree(body);
        function.addChildToBack(replacement);
        t.reportCodeChange();
      }
    }
docs: TODO(bradfordcsmith): This should be separated from this pass.