Function ref

packages/react-dom/src/__tests__/refs-test.js:536–543

    let current = null;

    const ref = r => {
      current = r;
      createCalls++;
      return () => {
        current = null;
        cleanupCalls++;
      };
    };

    await act(async () => {
      root.render(<ImperativeHandleComponent name="Alice" ref={ref} />);