**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

applySimpleSelectorScope: function(selector, scopeSelector) {
        if (selector.match(polyfillHostRe)) {
          selector = selector.replace(polyfillHostNoCombinator, scopeSelector);
          return selector.replace(polyfillHostRe, scopeSelector + " ");
        } else {
          return scopeSelector + " " + selector;
        }
      },
      applyStrictSelectorScope: function(selector, scopeSelector) {
        scopeSelector = scopeSelector.replace(/\[is=([^\]]*)\]/g, "$1");
        var splits = [ " ", ">", "+", "~" ], scoped = selector, attrName = "[" + scopeSelector + "]";
        splits.forEach(function(sep) {
          var parts = scoped.split(sep);
          scoped = parts.map(function(p) {
            var t = p.trim().replace(polyfillHostRe, "");
            if (t && splits.indexOf(t) < 0 && t.indexOf(attrName) < 0) {
              p = t.replace(/([^:]*)(:*)(.*)/, "$1" + attrName + "$2$3");
            }
            return p;
          }).join(sep);
        });
        return scoped;
      },
      insertPolyfillHostInCssText: function(selector) {
        return selector.replace(colonHostContextRe, polyfillHostContext).replace(colonHostRe, polyfillHost);
      },
      propertiesFromRule: function(rule) {
        var cssText = rule.style.cssText;
        if (rule.style.content && !rule.style.content.match(/['"]+|attr/)) {
          cssText = cssText.replace(/content:[^;]*;/g, "content: '" + rule.style.content + "';");