<!--
  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

switch (element.type) {
        case 'div':
          return mockDivInstance;
        case 'input':
          return mockInputInstance;
        case 'li':
          return mockListItemInstance;
        case 'a':
          return mockAnchorInstance;
        default:
          return {};
      }
    }
    ReactTestRenderer.create(<div ref={r => log.push(r)} />, {createNodeMock});
    ReactTestRenderer.create(<input ref={r => log.push(r)} />, {
      createNodeMock,
    });
    ReactTestRenderer.create(
      <div>
        <span>
          <ul>
            <li ref={r => log.push(r)} />
          </ul>
          <ul>
            <li ref={r => log.push(r)} />
            <li ref={r => log.push(r)} />
          </ul>
        </span>
      </div>,