<!--
  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

render() {
        return null;
      }
    }
    const renderer = ReactTestRenderer.create(<Foo />);
    const tree = renderer.toTree();
    expect(tree.instance).toBeInstanceOf(Foo);
    cleanNodeOrArray(tree);
    expect(tree).toEqual({
      type: Foo,
      nodeType: 'component',
      props: {},
      instance: null,
      rendered: null,
    });
  });
  it('toTree() handles simple components that return arrays', () => {
    const Foo = ({children}) => children;
    const renderer = ReactTestRenderer.create(
      <Foo>
        <div>One</div>
        <div>Two</div>
      </Foo>,