**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

if (!node) {
            if (extendsOption) {
              return document.createElement(extendsOption, tagName);
            } else {
              return document.createElement(tagName);
            }
          }
          setWrapper(node, this);
        }
        CustomElementConstructor.prototype = prototype;
        CustomElementConstructor.prototype.constructor = CustomElementConstructor;
        scope.constructorTable.set(newPrototype, CustomElementConstructor);
        scope.nativePrototypeTable.set(prototype, newPrototype);
        var nativeConstructor = originalRegisterElement.call(unwrap(this), tagName, p);
        return CustomElementConstructor;
      };
      forwardMethodsToWrapper([ window.HTMLDocument || window.Document ], [ "registerElement" ]);
    }
    forwardMethodsToWrapper([ window.HTMLBodyElement, window.HTMLDocument || window.Document, window.HTMLHeadElement, window.HTMLHtmlElement ], [ "appendChild", "compareDocumentPosition", "contains", "getElementsByClassName", "getElementsByTagName", "getElementsByTagNameNS", "insertBefore", "querySelector", "querySelectorAll", "removeChild", "replaceChild" ].concat(matchesNames));
    forwardMethodsToWrapper([ window.HTMLDocument || window.Document ], [ "adoptNode", "importNode", "contains", "createComment", "createDocumentFragment", "createElement", "createElementNS", "createEvent", "createEventNS", "createRange", "createTextNode", "elementFromPoint", "getElementById", "getElementsByName", "getSelection" ]);
    mixin(Document.prototype, GetElementsByInterface);
    mixin(Document.prototype, ParentNodeInterface);
    mixin(Document.prototype, SelectorsInterface);
    mixin(Document.prototype, {
      get implementation() {