<!--
  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

this.setState({x: 7});
      }
    }
    const Child = () => {
      renders++;
      return <moo />;
    };
    const Null = () => {
      renders++;
      return null;
    };
    const renderer = ReactTestRenderer.create(<Component />);
    expect(renderer.toJSON()).toEqual({
      type: 'div',
      props: {className: 'purple'},
      children: ['7', {type: 'moo', props: {}, children: null}],
    });
    expect(renders).toBe(6);
  });
  it('exposes the instance', () => {
    class Mouse extends React.Component {