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

renderer.update(42);
    expect(renderer.toJSON()).toEqual('42');
    renderer.update([42, 'world']);
    expect(renderer.toJSON()).toEqual(['42', 'world']);
  });
  it('can render and update root fragments', () => {
    const Component = props => props.children;
    const renderer = ReactTestRenderer.create([
      <Component key="a">Hi</Component>,
      <Component key="b">Bye</Component>,
    ]);
    expect(renderer.toJSON()).toEqual(['Hi', 'Bye']);
    renderer.update(<div />);
    expect(renderer.toJSON()).toEqual({
      type: 'div',
      children: null,
      props: {},
    });
    renderer.update([<div key="a">goodbye</div>, 'world']);
    expect(renderer.toJSON()).toEqual([
      {
        type: 'div',
        children: ['goodbye'],
        props: {},
      },
      'world',
    ]);
  });
});