**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

while (i > 0 || j > 0) {
          if (i == 0) {
            edits.push(EDIT_ADD);
            j--;
            continue;
          }
          if (j == 0) {
            edits.push(EDIT_DELETE);
            i--;
            continue;
          }
          var northWest = distances[i - 1][j - 1];
          var west = distances[i - 1][j];
          var north = distances[i][j - 1];
          var min;
          if (west < north) min = west < northWest ? west : northWest; else min = north < northWest ? north : northWest;
          if (min == northWest) {
            if (northWest == current) {
              edits.push(EDIT_LEAVE);
            } else {
              edits.push(EDIT_UPDATE);
              current = northWest;
            }
            i--;
            j--;
          } else if (min == west) {
            edits.push(EDIT_DELETE);
            i--;
            current = west;
          } else {