**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

ArraySplice.prototype = {
      calcEditDistances: function(current, currentStart, currentEnd, old, oldStart, oldEnd) {
        var rowCount = oldEnd - oldStart + 1;
        var columnCount = currentEnd - currentStart + 1;
        var distances = new Array(rowCount);
        for (var i = 0; i < rowCount; i++) {
          distances[i] = new Array(columnCount);
          distances[i][0] = i;
        }
        for (var j = 0; j < columnCount; j++) distances[0][j] = j;
        for (var i = 1; i < rowCount; i++) {
          for (var j = 1; j < columnCount; j++) {
            if (this.equals(current[currentStart + j - 1], old[oldStart + i - 1])) distances[i][j] = distances[i - 1][j - 1]; else {
              var north = distances[i - 1][j] + 1;
              var west = distances[i][j - 1] + 1;
              distances[i][j] = north < west ? north : west;
            }
          }
        }
        return distances;
      },
      spliceOperationsFromEditDistances: function(distances) {
        var i = distances.length - 1;
        var j = distances[0].length - 1;
        var current = distances[i][j];
        var edits = [];