<!--
  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

componentWillUnmount() {
        count++;
      }
      render() {
        return <div />;
      }
    }
    const inst = ReactTestRenderer.create(
      <div>
        <Foo />
      </div>,
      {
        createNodeMock: () => 'foo',
      },
    );
    expect(() => inst.unmount()).not.toThrow();
    expect(count).toEqual(1);
  });
  it('supports updates when using refs', () => {
    const log = [];
    const createNodeMock = element => {
      log.push(element.type);
      return element.type;
    };
    class Foo extends React.Component {