**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

shimCssText: function(cssText, selector) {
        cssText = this.insertDirectives(cssText);
        return this.scopeCssText(cssText, selector);
      },
      makeScopeSelector: function(name, typeExtension) {
        if (name) {
          return typeExtension ? "[is=" + name + "]" : name;
        }
        return "";
      },
      isTypeExtension: function(extendsName) {
        return extendsName && extendsName.indexOf("-") < 0;
      },
      prepareRoot: function(root, name, extendsName) {
        var def = this.registerRoot(root, name, extendsName);
        this.replaceTextInStyles(def.rootStyles, this.insertDirectives);
        this.removeStyles(root, def.rootStyles);
        if (this.strictStyling) {
          this.applyScopeToContent(root, name);
        }
        return def.scopeStyles;
      },
      removeStyles: function(root, styles) {
        for (var i = 0, l = styles.length, s; i < l && (s = styles[i]); i++) {
          s.parentNode.removeChild(s);
        }
      },
      registerRoot: function(root, name, extendsName) {
        var def = this.registry[name] = {
          root: root,