<!--
  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

});
  it('updates types', () => {
    const renderer = ReactTestRenderer.create(<div>mouse</div>);
    expect(renderer.toJSON()).toEqual({
      type: 'div',
      props: {},
      children: ['mouse'],
    });
    renderer.update(<span>mice</span>);
    expect(renderer.toJSON()).toEqual({
      type: 'span',
      props: {},
      children: ['mice'],
    });
  });
  it('updates children', () => {
    const renderer = ReactTestRenderer.create(
      <div>
        <span key="a">A</span>
        <span key="b">B</span>
        <span key="c">C</span>
      </div>,
    );
    expect(renderer.toJSON()).toEqual({
      type: 'div',
      props: {},
      children: [
        {type: 'span', props: {}, children: ['A']},
        {type: 'span', props: {}, children: ['B']},
        {type: 'span', props: {}, children: ['C']},