**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

parts.forEach(function(p) {
          p = p.trim();
          if (this.selectorNeedsScoping(p, scopeSelector)) {
            p = strict && !p.match(polyfillHostNoCombinator) ? this.applyStrictSelectorScope(p, scopeSelector) : this.applySelectorScope(p, scopeSelector);
          }
          r.push(p);
        }, this);
        return r.join(", ");
      },
      selectorNeedsScoping: function(selector, scopeSelector) {
        if (Array.isArray(scopeSelector)) {
          return true;
        }
        var re = this.makeScopeMatcher(scopeSelector);
        return !selector.match(re);
      },
      makeScopeMatcher: function(scopeSelector) {
        scopeSelector = scopeSelector.replace(/\[/g, "\\[").replace(/\[/g, "\\]");
        return new RegExp("^(" + scopeSelector + ")" + selectorReSuffix, "m");
      },
      applySelectorScope: function(selector, selectorScope) {
        return Array.isArray(selectorScope) ? this.applySelectorScopeList(selector, selectorScope) : this.applySimpleSelectorScope(selector, selectorScope);
      },
      applySelectorScopeList: function(selector, scopeSelectorList) {
        var r = [];
        for (var i = 0, s; s = scopeSelectorList[i]; i++) {
          r.push(this.applySimpleSelectorScope(selector, s));
        }
        return r.join(", ");
      },