**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

cssText += this.propertiesFromRule(rule) + "\n}\n\n";
            } else if (rule.type === CSSRule.MEDIA_RULE) {
              cssText += "@media " + rule.media.mediaText + " {\n";
              cssText += this.scopeRules(rule.cssRules, scopeSelector);
              cssText += "\n}\n\n";
            } else {
              try {
                if (rule.cssText) {
                  cssText += rule.cssText + "\n\n";
                }
              } catch (x) {
                if (rule.type === CSSRule.KEYFRAMES_RULE && rule.cssRules) {
                  cssText += this.ieSafeCssTextFromKeyFrameRule(rule);
                }
              }
            }
          }, this);
        }
        return cssText;
      },
      ieSafeCssTextFromKeyFrameRule: function(rule) {
        var cssText = "@keyframes " + rule.name + " {";
        Array.prototype.forEach.call(rule.cssRules, function(rule) {
          cssText += " " + rule.keyText + " {" + rule.style.cssText + "}";
        });
        cssText += " }";
        return cssText;
      },
      scopeSelector: function(selector, scopeSelector, strict) {
        var r = [], parts = selector.split(",");