Function onHeaders

packages/react-dom/src/__tests__/ReactDOMFizzServer-test.js:3708–3710

(x)

  it('provides headers after initial work if onHeaders option used', async () => {
    let headers = null;
    function onHeaders(x) {
      headers = x;
    }

    function Preloads() {
      ReactDOM.preload('font2', {as: 'font'});