**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

},
      insertDirectives: function(cssText) {
        cssText = this.insertPolyfillDirectivesInCssText(cssText);
        return this.insertPolyfillRulesInCssText(cssText);
      },
      insertPolyfillDirectivesInCssText: function(cssText) {
        cssText = cssText.replace(cssCommentNextSelectorRe, function(match, p1) {
          return p1.slice(0, -2) + "{";
        });
        return cssText.replace(cssContentNextSelectorRe, function(match, p1) {
          return p1 + " {";
        });
      },
      insertPolyfillRulesInCssText: function(cssText) {
        cssText = cssText.replace(cssCommentRuleRe, function(match, p1) {
          return p1.slice(0, -1);
        });
        return cssText.replace(cssContentRuleRe, function(match, p1, p2, p3) {
          var rule = match.replace(p1, "").replace(p2, "");
          return p3 + rule;
        });
      },
      scopeCssText: function(cssText, scopeSelector) {
        var unscoped = this.extractUnscopedRulesFromCssText(cssText);
        cssText = this.insertPolyfillHostInCssText(cssText);
        cssText = this.convertColonHost(cssText);
        cssText = this.convertColonHostContext(cssText);
        cssText = this.convertShadowDOMSelectors(cssText);
        if (scopeSelector) {
          var self = this, cssText;