<!--
  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

if (descend && node.child !== null) {
        node.child.return = node;
        node = node.child;
        continue;
      }
      while (node.sibling === null) {
        if (node.return === startingNode) {
          break outer;
        }
        node = (node.return: any);
      }
      (node.sibling: any).return = node.return;
      node = (node.sibling: any);
    }
    return children;
  }
  
  find(predicate: Predicate): ReactTestInstance {
    return expectOne(
      this.findAll(predicate, {deep: false}),
      `matching custom predicate: ${predicate.toString()}`,
    );
  }
  findByType(type: any): ReactTestInstance {
    return expectOne(
      this.findAllByType(type, {deep: false}),
      `with node type: "${type.displayName || type.name}"`,
    );
  }
  findByProps(props: Object): ReactTestInstance {