**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 clone = style.ownerDocument.createElement("style");
    clone.textContent = style.textContent;
    path.resolveUrlsInStyle(clone);
    return clone;
  }
  scope.parser = importParser;
  scope.IMPORT_SELECTOR = IMPORT_SELECTOR;
});
HTMLImports.addModule(function(scope) {
  var flags = scope.flags;
  var IMPORT_LINK_TYPE = scope.IMPORT_LINK_TYPE;
  var IMPORT_SELECTOR = scope.IMPORT_SELECTOR;
  var rootDocument = scope.rootDocument;
  var Loader = scope.Loader;
  var Observer = scope.Observer;
  var parser = scope.parser;
  var importer = {
    documents: {},
    documentPreloadSelectors: IMPORT_SELECTOR,
    importsPreloadSelectors: [ IMPORT_SELECTOR ].join(","),
    loadNode: function(node) {
      importLoader.addNode(node);
    },
    loadSubtree: function(parent) {
      var nodes = this.marshalNodes(parent);
      importLoader.addNodes(nodes);