<!--
  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

}
    parentInstance.children.push(child);
  },
  finalizeInitialChildren(
    testElement: Instance,
    type: string,
    props: Props,
    rootContainerInstance: Container,
  ): boolean {
    return false;
  },
  prepareUpdate(
    testElement: Instance,
    type: string,
    oldProps: Props,
    newProps: Props,
    rootContainerInstance: Container,
    hostContext: Object,
  ): null | {} {
    return UPDATE_SIGNAL;
  },
  shouldSetTextContent(type: string, props: Props): boolean {
    return false;
  },
  shouldDeprioritizeSubtree(type: string, props: Props): boolean {
    return false;
  },
  createTextInstance(
    text: string,
    rootContainerInstance: Container,