<!--
  Note: if the issue is about documentation or the website, please file it at:
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**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

const children = [];
    const startingNode = this._currentFiber();
    let node: Fiber = startingNode;
    if (node.child === null) {
      return children;
    }
    node.child.return = node;
    node = node.child;
    outer: while (true) {
      let descend = false;
      switch (node.tag) {
        case FunctionalComponent:
        case ClassComponent:
        case HostComponent:
          children.push(wrapFiber(node));
          break;
        case HostText:
          children.push('' + node.memoizedProps);
          break;
        case Fragment:
          descend = true;
          break;
        default:
          invariant(
            false,
            'Unsupported component type %s in test renderer. ' +
              'This is probably a bug in React.',
            node.tag,
          );
      }