**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

},
    marshalNodes: function(parent) {
      return parent.querySelectorAll(this.loadSelectorsForNode(parent));
    },
    loadSelectorsForNode: function(node) {
      var doc = node.ownerDocument || node;
      return doc === rootDocument ? this.documentPreloadSelectors : this.importsPreloadSelectors;
    },
    loaded: function(url, elt, resource, err, redirectedUrl) {
      flags.load && console.log("loaded", url, elt);
      elt.__resource = resource;
      elt.__error = err;
      if (isImportLink(elt)) {
        var doc = this.documents[url];
        if (doc === undefined) {
          doc = err ? null : makeDocument(resource, redirectedUrl || url);
          if (doc) {
            doc.__importLink = elt;
            this.bootDocument(doc);
          }
          this.documents[url] = doc;
        }
        elt.import = doc;
      }
      parser.parseNext();
    },
    bootDocument: function(doc) {
      this.loadSubtree(doc);
      this.observer.observe(doc);
      parser.parseNext();