**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

var implementation = implementationTable.get(this);
        if (implementation) return implementation;
        implementation = new DOMImplementation(unwrap(this).implementation);
        implementationTable.set(this, implementation);
        return implementation;
      },
      get defaultView() {
        return wrap(unwrap(this).defaultView);
      }
    });
    registerWrapper(window.Document, Document, document.implementation.createHTMLDocument(""));
    if (window.HTMLDocument) registerWrapper(window.HTMLDocument, Document);
    wrapEventTargetMethods([ window.HTMLBodyElement, window.HTMLDocument || window.Document, window.HTMLHeadElement ]);
    function DOMImplementation(impl) {
      setWrapper(impl, this);
    }
    function wrapImplMethod(constructor, name) {
      var original = document.implementation[name];
      constructor.prototype[name] = function() {
        return wrap(original.apply(unsafeUnwrap(this), arguments));
      };
    }
    function forwardImplMethod(constructor, name) {