<!--
  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

let wrapper = fiberToWrapper.get(fiber);
  if (wrapper === undefined && fiber.alternate !== null) {
    wrapper = fiberToWrapper.get(fiber.alternate);
  }
  if (wrapper === undefined) {
    wrapper = new ReactTestInstance(fiber);
    fiberToWrapper.set(fiber, wrapper);
  }
  return wrapper;
}
const validWrapperTypes = new Set([
  FunctionalComponent,
  ClassComponent,
  HostComponent,
]);
class ReactTestInstance {
  _fiber: Fiber;
  _currentFiber(): Fiber {
    
    const fiber = findCurrentFiberUsingSlowPath(this._fiber);
    invariant(
      fiber !== null,
      "Can't read from currently-mounting component. This error is likely " +
        'caused by a bug in React. Please file an issue.',
    );
    return fiber;
  }
  constructor(fiber: Fiber) {