<!--
  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

{createNodeMock, foobar: true},
    );
    ReactTestRenderer.create(<Foo />, {createNodeMock});
    ReactTestRenderer.create(<div ref={r => log.push(r)} />);
    ReactTestRenderer.create(<div ref={r => log.push(r)} />, {});
    expect(log).toEqual([
      mockDivInstance,
      mockInputInstance,
      mockListItemInstance,
      mockListItemInstance,
      mockListItemInstance,
      mockAnchorInstance,
      null,
      null,
    ]);
  });
  it('supports unmounting when using refs', () => {
    class Foo extends React.Component {
      render() {
        return <div ref="foo" />;
      }
    }
    const inst = ReactTestRenderer.create(<Foo />, {
      createNodeMock: () => 'foo',
    });
    expect(() => inst.unmount()).not.toThrow();
  });
  it('supports unmounting inner instances', () => {
    let count = 0;
    class Foo extends React.Component {