**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

var listeners = listenersTable.get(currentTarget);
      if (!listeners) return true;
      var target = overrideTarget || eventRetargetting(eventPath, currentTarget);
      if (target === currentTarget) {
        if (phase === CAPTURING_PHASE) return true;
        if (phase === BUBBLING_PHASE) phase = AT_TARGET;
      } else if (phase === BUBBLING_PHASE && !event.bubbles) {
        return true;
      }
      if ("relatedTarget" in event) {
        var originalEvent = unwrap(event);
        var unwrappedRelatedTarget = originalEvent.relatedTarget;
        if (unwrappedRelatedTarget) {
          if (unwrappedRelatedTarget instanceof Object && unwrappedRelatedTarget.addEventListener) {
            var relatedTarget = wrap(unwrappedRelatedTarget);
            var adjusted = relatedTargetResolution(event, currentTarget, relatedTarget);
            if (adjusted === target) return true;
          } else {
            adjusted = null;
          }
          relatedTargetTable.set(event, adjusted);
        }
      }
      eventPhaseTable.set(event, phase);
      var type = event.type;
      var anyRemoved = false;