**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

resolveUrlsInCssText: function(cssText, urlObj) {
      var r = this.replaceUrls(cssText, urlObj, CSS_URL_REGEXP);
      r = this.replaceUrls(r, urlObj, CSS_IMPORT_REGEXP);
      return r;
    },
    replaceUrls: function(text, urlObj, regexp) {
      return text.replace(regexp, function(m, pre, url, post) {
        var urlPath = url.replace(/["']/g, "");
        urlObj.href = urlPath;
        urlPath = urlObj.href;
        return pre + "'" + urlPath + "'" + post;
      });
    }
  };
  scope.path = path;
});
HTMLImports.addModule(function(scope) {
  xhr = {
    async: true,
    ok: function(request) {
      return request.status >= 200 && request.status < 300 || request.status === 304 || request.status === 0;
    },
    load: function(url, next, nextContext) {
      var request = new XMLHttpRequest();
      if (scope.flags.debug || scope.flags.bust) {
        url += "?" + Math.random();
      }
      request.open("GET", url, xhr.async);
      request.addEventListener("readystatechange", function(e) {
        if (request.readyState === 4) {