<!--
  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

render() {
        return this.props.useDiv ? <div ref="foo" /> : <span ref="foo" />;
      }
    }
    const inst = ReactTestRenderer.create(<Foo useDiv={true} />, {
      createNodeMock,
    });
    inst.update(<Foo useDiv={false} />);
    expect(log).toEqual(['div', 'span']);
  });
  it('supports error boundaries', () => {
    const log = [];
    class Angry extends React.Component {
      render() {
        log.push('Angry render');
        throw new Error('Please, do not render me.');
      }
      componentDidMount() {
        log.push('Angry componentDidMount');
      }
      componentWillUnmount() {
        log.push('Angry componentWillUnmount');
      }
    }
    class Boundary extends React.Component {