<!--
  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

case FunctionalComponent: 
      return {
        nodeType: 'component',
        type: node.type,
        props: {...node.memoizedProps},
        instance: null,
        rendered: hasSiblings(node.child)
          ? nodeAndSiblingsTrees(node.child)
          : toTree(node.child),
      };
    case HostComponent: 
      return {
        nodeType: 'host',
        type: node.type,
        props: {...node.memoizedProps},
        instance: null, 
        rendered: nodeAndSiblingsTrees(node.child),
      };
    case HostText: 
      return node.stateNode.text;
    default:
      invariant(
        false,
        'toTree() does not yet know how to handle nodes with tag=%s',
        node.tag,
      );
  }
}
const fiberToWrapper = new WeakMap();
function wrapFiber(fiber: Fiber): ReactTestInstance {