<!--
  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

},
};
function toJSON(inst: Instance | TextInstance): ReactTestRendererNode {
  switch (inst.tag) {
    case 'TEXT':
      return inst.text;
    case 'INSTANCE':
      
      
      
      const {children, ...props} = inst.props;
      
      let renderedChildren = null;
      if (inst.children && inst.children.length) {
        renderedChildren = inst.children.map(toJSON);
      }
      const json: ReactTestRendererJSON = {
        type: inst.type,
        props: props,
        children: renderedChildren,
      };
      Object.defineProperty(json, '$$typeof', {
        value: Symbol.for('react.test.json'),
      });
      return json;
    default:
      throw new Error(`Unexpected node type in toJSON: ${inst.tag}`);
  }
}
function nodeAndSiblingsTrees(nodeWithSibling: ?Fiber) {