**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

dynamicElements: [],
    parseNext: function() {
      var next = this.nextToParse();
      if (next) {
        this.parse(next);
      }
    },
    parse: function(elt) {
      if (this.isParsed(elt)) {
        flags.parse && console.log("[%s] is already parsed", elt.localName);
        return;
      }
      var fn = this[this.map[elt.localName]];
      if (fn) {
        this.markParsing(elt);
        fn.call(this, elt);
      }
    },
    parseDynamic: function(elt, quiet) {
      this.dynamicElements.push(elt);
      if (!quiet) {
        this.parseNext();
      }
    },
    markParsing: function(elt) {
      flags.parse && console.log("parsing", elt);
      this.parsingElement = elt;
    },
    markParsingComplete: function(elt) {
      elt.__importParsed = true;