**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

return this.parentNode_ !== undefined ? this.parentNode_ : wrap(unsafeUnwrap(this).parentNode);
      },
      get firstChild() {
        return this.firstChild_ !== undefined ? this.firstChild_ : wrap(unsafeUnwrap(this).firstChild);
      },
      get lastChild() {
        return this.lastChild_ !== undefined ? this.lastChild_ : wrap(unsafeUnwrap(this).lastChild);
      },
      get nextSibling() {
        return this.nextSibling_ !== undefined ? this.nextSibling_ : wrap(unsafeUnwrap(this).nextSibling);
      },
      get previousSibling() {
        return this.previousSibling_ !== undefined ? this.previousSibling_ : wrap(unsafeUnwrap(this).previousSibling);
      },
      get parentElement() {
        var p = this.parentNode;
        while (p && p.nodeType !== Node.ELEMENT_NODE) {
          p = p.parentNode;
        }
        return p;
      },
      get textContent() {