**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

},
    nextToParse: function() {
      this._mayParse = [];
      return !this.parsingElement && (this.nextToParseInDoc(rootDocument) || this.nextToParseDynamic());
    },
    nextToParseInDoc: function(doc, link) {
      if (doc && this._mayParse.indexOf(doc) < 0) {
        this._mayParse.push(doc);
        var nodes = doc.querySelectorAll(this.parseSelectorsForNode(doc));
        for (var i = 0, l = nodes.length, p = 0, n; i < l && (n = nodes[i]); i++) {
          if (!this.isParsed(n)) {
            if (this.hasResource(n)) {
              return nodeIsImport(n) ? this.nextToParseInDoc(n.import, n) : n;
            } else {
              return;
            }
          }
        }
      }
      return link;
    },
    nextToParseDynamic: function() {
      return this.dynamicElements[0];
    },
    parseSelectorsForNode: function(node) {
      var doc = node.ownerDocument || node;
      return doc === rootDocument ? this.documentSelectors : this.importsSelectors;
    },
    isParsed: function(node) {
      return node.__importParsed;