**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

if (this.shouldParseNode(n) && parsed) {
          parser.parseDynamic(n, loading);
        }
      }
    },
    shouldLoadNode: function(node) {
      return node.nodeType === 1 && matches.call(node, importer.loadSelectorsForNode(node));
    },
    shouldParseNode: function(node) {
      return node.nodeType === 1 && matches.call(node, parser.parseSelectorsForNode(node));
    }
  };
  importer.observer.addCallback = dynamic.added.bind(dynamic);
  var matches = HTMLElement.prototype.matches || HTMLElement.prototype.matchesSelector || HTMLElement.prototype.webkitMatchesSelector || HTMLElement.prototype.mozMatchesSelector || HTMLElement.prototype.msMatchesSelector;
});
(function(scope) {
  initializeModules = scope.initializeModules;
  if (scope.useNative) {
    return;
  }
  if (typeof window.CustomEvent !== "function") {
    window.CustomEvent = function(inType, dictionary) {
      var e = document.createEvent("HTMLEvents");
      e.initEvent(inType, dictionary.bubbles === false ? false : true, dictionary.cancelable === false ? false : true, dictionary.detail);
      return e;
    };
  }
  initializeModules();
  var rootDocument = scope.rootDocument;
  function bootstrap() {