<!--
  Note: if the issue is about documentation or the website, please file it at:
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**Do you want to request a *feature* or report a *bug*?**

bug.

**What is the current behavior?**

```js
  renderer.create(
    <Fragment>
      <Fragment>foo</Fragment>
    </Fragment>
  )
  .toTree()
```

> Invariant Violation: toTree() does not yet know how to handle nodes with tag=10

https://codesandbox.io/s/olky92m01y

**What is the expected behavior?**

returns a tree.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**

16.2.0

render() {
        return <div ref="baz" />;
      }
    }
    ReactTestRenderer.create(<Baz />);
    expect(() => ReactTestRenderer.create(<Foo />)).toWarnDev(
      'Warning: Stateless function components cannot be given refs. Attempts ' +
        'to access this ref will fail.\n\nCheck the render method of `Foo`.\n' +
        '    in Bar (at **)\n' +
        '    in Foo (at **)',
    );
  });
  it('allows an optional createNodeMock function', () => {
    const mockDivInstance = {appendChild: () => {}};
    const mockInputInstance = {focus: () => {}};
    const mockListItemInstance = {click: () => {}};
    const mockAnchorInstance = {hover: () => {}};
    const log = [];
    class Foo extends React.Component {
      componentDidMount() {
        log.push(this.refs.bar);
      }
      render() {
        return <a ref="bar">Hello, world</a>;
      }
    }
    function createNodeMock(element) {