<!--
  Note: if the issue is about documentation or the website, please file it at:
  https://github.com/reactjs/reactjs.org/issues/new
-->

**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

return null;
    }
    const renderer = ReactTestRenderer.create(<Empty />);
    expect(renderer.toJSON()).toEqual(null);
  });
  it('exposes a type flag', () => {
    function Link() {
      return <a role="link" />;
    }
    const renderer = ReactTestRenderer.create(<Link />);
    const object = renderer.toJSON();
    expect(object.$$typeof).toBe(Symbol.for('react.test.json'));
    
    for (const key in object) {
      if (object.hasOwnProperty(key)) {
        expect(key).not.toBe('$$typeof');
      }
    }
  });
  it('can render a composite component', () => {
    class Component extends React.Component {