**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

edits.push(EDIT_ADD);
            j--;
            current = north;
          }
        }
        edits.reverse();
        return edits;
      },
      calcSplices: function(current, currentStart, currentEnd, old, oldStart, oldEnd) {
        var prefixCount = 0;
        var suffixCount = 0;
        var minLength = Math.min(currentEnd - currentStart, oldEnd - oldStart);
        if (currentStart == 0 && oldStart == 0) prefixCount = this.sharedPrefix(current, old, minLength);
        if (currentEnd == current.length && oldEnd == old.length) suffixCount = this.sharedSuffix(current, old, minLength - prefixCount);
        currentStart += prefixCount;
        oldStart += prefixCount;
        currentEnd -= suffixCount;
        oldEnd -= suffixCount;
        if (currentEnd - currentStart == 0 && oldEnd - oldStart == 0) return [];
        if (currentStart == currentEnd) {
          var splice = newSplice(currentStart, [], 0);
          while (oldStart < oldEnd) splice.removed.push(old[oldStart++]);
          return [ splice ];
        } else if (oldStart == oldEnd) return [ newSplice(currentStart, [], currentEnd - currentStart) ];
        var ops = this.spliceOperationsFromEditDistances(this.calcEditDistances(current, currentStart, currentEnd, old, oldStart, oldEnd));
        var splice = undefined;
        var splices = [];
        var index = currentStart;
        var oldIndex = oldStart;
        for (var i = 0; i < ops.length; i++) {