**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

head.insertBefore(getSheet(), head.childNodes[0]);
      document.addEventListener("DOMContentLoaded", function() {
        var urlResolver = scope.urlResolver;
        if (window.HTMLImports && !HTMLImports.useNative) {
          var SHIM_SHEET_SELECTOR = "link[rel=stylesheet]" + "[" + SHIM_ATTRIBUTE + "]";
          var SHIM_STYLE_SELECTOR = "style[" + SHIM_ATTRIBUTE + "]";
          HTMLImports.importer.documentPreloadSelectors += "," + SHIM_SHEET_SELECTOR;
          HTMLImports.importer.importsPreloadSelectors += "," + SHIM_SHEET_SELECTOR;
          HTMLImports.parser.documentSelectors = [ HTMLImports.parser.documentSelectors, SHIM_SHEET_SELECTOR, SHIM_STYLE_SELECTOR ].join(",");
          var originalParseGeneric = HTMLImports.parser.parseGeneric;
          HTMLImports.parser.parseGeneric = function(elt) {
            if (elt[SHIMMED_ATTRIBUTE]) {
              return;
            }
            var style = elt.__importElement || elt;
            if (!style.hasAttribute(SHIM_ATTRIBUTE)) {
              originalParseGeneric.call(this, elt);
              return;
            }
            if (elt.__resource) {
              style = elt.ownerDocument.createElement("style");
              style.textContent = elt.__resource;
            }
            HTMLImports.path.resolveUrlsInStyle(style);
            style.textContent = ShadowCSS.shimStyle(style);
            style.removeAttribute(SHIM_ATTRIBUTE, "");
            style.setAttribute(SHIMMED_ATTRIBUTE, "");
            style[SHIMMED_ATTRIBUTE] = true;
            if (style.parentNode !== head) {
              if (elt.parentNode === head) {