**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

r.push(partReplacer(p1, p, p3));
            }
            return r.join(",");
          } else {
            return p1 + p3;
          }
        });
      },
      colonHostContextPartReplacer: function(host, part, suffix) {
        if (part.match(polyfillHost)) {
          return this.colonHostPartReplacer(host, part, suffix);
        } else {
          return host + part + suffix + ", " + part + " " + host + suffix;
        }
      },
      colonHostPartReplacer: function(host, part, suffix) {
        return host + part.replace(polyfillHost, "") + suffix;
      },
      convertShadowDOMSelectors: function(cssText) {
        for (var i = 0; i < shadowDOMSelectorsRe.length; i++) {
          cssText = cssText.replace(shadowDOMSelectorsRe[i], " ");
        }
        return cssText;
      },
      scopeRules: function(cssRules, scopeSelector) {
        var cssText = "";
        if (cssRules) {
          Array.prototype.forEach.call(cssRules, function(rule) {
            if (rule.selectorText && (rule.style && rule.style.cssText !== undefined)) {
              cssText += this.scopeSelector(rule.selectorText, scopeSelector, this.strictStyling) + " {\n	";