**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 getOuterHTML(this, this.parentNode);
      },
      set outerHTML(value) {
        var p = this.parentNode;
        if (p) {
          p.invalidateShadowRenderer();
          var df = frag(p, value);
          p.replaceChild(df, this);
        }
      },
      insertAdjacentHTML: function(position, text) {
        var contextElement, refNode;
        switch (String(position).toLowerCase()) {
         case "beforebegin":
          contextElement = this.parentNode;
          refNode = this;
          break;
         case "afterend":
          contextElement = this.parentNode;
          refNode = this.nextSibling;
          break;
         case "afterbegin":
          contextElement = this;
          refNode = this.firstChild;
          break;
         case "beforeend":
          contextElement = this;