**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 (supportsEventConstructors) return new OriginalEvent(type, unwrapOptions(options));
      var event = unwrap(document.createEvent(name));
      var defaultDict = defaultInitDicts[name];
      var args = [ type ];
      Object.keys(defaultDict).forEach(function(key) {
        var v = options != null && key in options ? options[key] : defaultDict[key];
        if (key === "relatedTarget") v = unwrap(v);
        args.push(v);
      });
      event["init" + name].apply(event, args);
      return event;
    }
    if (!supportsEventConstructors) {
      var configureEventConstructor = function(name, initDict, superName) {
        if (superName) {
          var superDict = defaultInitDicts[superName];
          initDict = mixin(mixin({}, superDict), initDict);
        }
        defaultInitDicts[name] = initDict;