**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 (!options || !options.relatedTarget) return options;
      return Object.create(options, {
        relatedTarget: {
          value: unwrap(options.relatedTarget)
        }
      });
    }
    function registerGenericEvent(name, SuperEvent, prototype) {
      var OriginalEvent = window[name];
      var GenericEvent = function(type, options) {
        if (type instanceof OriginalEvent) setWrapper(type, this); else return wrap(constructEvent(OriginalEvent, name, type, options));
      };
      GenericEvent.prototype = Object.create(SuperEvent.prototype);
      if (prototype) mixin(GenericEvent.prototype, prototype);
      if (OriginalEvent) {
        try {
          registerWrapper(OriginalEvent, GenericEvent, new OriginalEvent("temp"));
        } catch (ex) {
          registerWrapper(OriginalEvent, GenericEvent, document.createEvent(name));
        }
      }
      return GenericEvent;
    }
    var UIEvent = registerGenericEvent("UIEvent", Event);
    var CustomEvent = registerGenericEvent("CustomEvent", Event);
    var relatedTargetProto = {
      get relatedTarget() {