**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

withCssRules(cssText, function(rules) {
            cssText = self.scopeRules(rules, scopeSelector);
          });
        }
        cssText = cssText + "\n" + unscoped;
        return cssText.trim();
      },
      extractUnscopedRulesFromCssText: function(cssText) {
        var r = "", m;
        while (m = cssCommentUnscopedRuleRe.exec(cssText)) {
          r += m[1].slice(0, -1) + "\n\n";
        }
        while (m = cssContentUnscopedRuleRe.exec(cssText)) {
          r += m[0].replace(m[2], "").replace(m[1], m[3]) + "\n\n";
        }
        return r;
      },
      convertColonHost: function(cssText) {
        return this.convertColonRule(cssText, cssColonHostRe, this.colonHostPartReplacer);
      },
      convertColonHostContext: function(cssText) {
        return this.convertColonRule(cssText, cssColonHostContextRe, this.colonHostContextPartReplacer);
      },
      convertColonRule: function(cssText, regExp, partReplacer) {
        return cssText.replace(regExp, function(m, p1, p2, p3) {
          p1 = polyfillHostNoCombinator;
          if (p2) {
            var parts = p2.split(","), r = [];
            for (var i = 0, l = parts.length, p; i < l && (p = parts[i]); i++) {
              p = p.trim();