**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 (handledEventsTable.get(originalEvent)) return;
      handledEventsTable.set(originalEvent, true);
      dispatchEvent(wrap(originalEvent), wrap(originalEvent.target));
      if (pendingError) {
        var err = pendingError;
        pendingError = null;
        throw err;
      }
    }
    function isLoadLikeEvent(event) {
      switch (event.type) {
       case "load":
       case "beforeunload":
       case "unload":
        return true;
      }
      return false;
    }
    function dispatchEvent(event, originalWrapperTarget) {
      if (currentlyDispatchingEvents.get(event)) throw new Error("InvalidStateError");
      currentlyDispatchingEvents.set(event, true);
      scope.renderAllPending();
      var eventPath;
      var overrideTarget;
      var win;
      if (isLoadLikeEvent(event) && !event.bubbles) {