**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 locationHeader = request.getResponseHeader("Location");
          var redirectedUrl = null;
          if (locationHeader) {
            var redirectedUrl = locationHeader.substr(0, 1) === "/" ? location.origin + locationHeader : locationHeader;
          }
          next.call(nextContext, !xhr.ok(request) && request, request.response || request.responseText, redirectedUrl);
        }
      });
      request.send();
      return request;
    },
    loadDocument: function(url, next, nextContext) {
      this.load(url, next, nextContext).responseType = "document";
    }
  };
  scope.xhr = xhr;
});
HTMLImports.addModule(function(scope) {
  var xhr = scope.xhr;
  var flags = scope.flags;
  var Loader = function(onLoad, onComplete) {
    this.cache = {};
    this.onload = onLoad;
    this.oncomplete = onComplete;
    this.inflight = 0;
    this.pending = {};
  };
  Loader.prototype = {
    addNodes: function(nodes) {
      this.inflight += nodes.length;