**Do you want to request a *feature* or report a *bug*?**
bug

**What is the current behavior?**
React 15.x was able to render within a DocumentFragment/shadowRoot, not possible with Fiber anymore

**DEMO**

https://www.webpackbin.com/bins/-KvPFG7-HGfQ34IgUxQt

```js
import React, {Component} from 'react'
import {render} from 'react-dom'


class MyComponent extends Component{
  render(){
    return <div>Hello From React !</div>
  }
}

class MyElement extends HTMLElement {
  static is = 'my-element'
  constructor(){
    super()
    const shadowRoot = this.attachShadow({mode:'open'})
    try {
      // this fails 
      render(<MyComponent/>, shadowRoot)
    } catch (e){
      console.error(e)
      shadowRoot.innerHTML = `
        <div style="color:red;">${e}</div>
        <div>see console log for stack trace</div>
       `
    }
  }
  connectedCallback(){
    console.log('MyElement mounted!')
  }
}

customElements.define(MyElement.is, MyElement)
```


related -> https://github.com/skatejs/renderer-react/issues/3

**What is the expected behavior?**
be able to render to shadow root like previous versions of React

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
React 16
Chrome

if (!nodes.length) return;
      var ownerDoc = owner.ownerDocument;
      if (ownerDoc === nodes[0].ownerDocument) return;
      for (var i = 0; i < nodes.length; i++) {
        scope.adoptNodeNoRemove(nodes[i], ownerDoc);
      }
    }
    function unwrapNodesForInsertion(owner, nodes) {
      adoptNodesIfNeeded(owner, nodes);
      var length = nodes.length;
      if (length === 1) return unwrap(nodes[0]);
      var df = unwrap(owner.ownerDocument.createDocumentFragment());
      for (var i = 0; i < length; i++) {
        df.appendChild(unwrap(nodes[i]));
      }
      return df;
    }
    function clearChildNodes(wrapper) {
      if (wrapper.firstChild_ !== undefined) {
        var child = wrapper.firstChild_;
        while (child) {
          var tmp = child;
          child = child.nextSibling_;
          tmp.parentNode_ = tmp.previousSibling_ = tmp.nextSibling_ = undefined;
        }
      }
      wrapper.firstChild_ = wrapper.lastChild_ = undefined;
    }
    function removeAllChildNodes(wrapper) {