**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

scope.invalidateRendererBasedOnAttribute(el, "class");
    }
    function DOMTokenList(impl, ownerElement) {
      setWrapper(impl, this);
      this.ownerElement_ = ownerElement;
    }
    DOMTokenList.prototype = {
      constructor: DOMTokenList,
      get length() {
        return unsafeUnwrap(this).length;
      },
      item: function(index) {
        return unsafeUnwrap(this).item(index);
      },
      contains: function(token) {
        return unsafeUnwrap(this).contains(token);
      },
      add: function() {
        unsafeUnwrap(this).add.apply(unsafeUnwrap(this), arguments);
        invalidateClass(this.ownerElement_);
      },
      remove: function() {
        unsafeUnwrap(this).remove.apply(unsafeUnwrap(this), arguments);
        invalidateClass(this.ownerElement_);
      },
      toggle: function(token) {
        var rv = unsafeUnwrap(this).toggle.apply(unsafeUnwrap(this), arguments);
        invalidateClass(this.ownerElement_);