**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

this.markDynamicParsingComplete(elt);
      if (elt.__importElement) {
        elt.__importElement.__importParsed = true;
        this.markDynamicParsingComplete(elt.__importElement);
      }
      this.parsingElement = null;
      flags.parse && console.log("completed", elt);
    },
    markDynamicParsingComplete: function(elt) {
      var i = this.dynamicElements.indexOf(elt);
      if (i >= 0) {
        this.dynamicElements.splice(i, 1);
      }
    },
    parseImport: function(elt) {
      if (HTMLImports.__importsParsingHook) {
        HTMLImports.__importsParsingHook(elt);
      }
      if (elt.import) {
        elt.import.__importParsed = true;
      }
      this.markParsingComplete(elt);
      if (elt.__resource && !elt.__error) {
        elt.dispatchEvent(new CustomEvent("load", {
          bubbles: false
        }));
      } else {
        elt.dispatchEvent(new CustomEvent("error", {
          bubbles: false
        }));