**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 names = getOwnPropertyNames(source);
      for (var i = 0; i < names.length; i++) {
        var name = names[i];
        if (name === "polymerBlackList_") continue;
        if (name in target) continue;
        if (source.polymerBlackList_ && source.polymerBlackList_[name]) continue;
        if (isFirefox) {
          source.__lookupGetter__(name);
        }
        var descriptor = getDescriptor(source, name);
        var getter, setter;
        if (allowMethod && typeof descriptor.value === "function") {
          target[name] = getMethod(name);
          continue;
        }
        var isEvent = isEventHandlerName(name);
        if (isEvent) getter = scope.getEventHandlerGetter(name); else getter = getGetter(name);
        if (descriptor.writable || descriptor.set || isBrokenSafari) {
          if (isEvent) setter = scope.getEventHandlerSetter(name); else setter = getSetter(name);
        }
        defineProperty(target, name, {
          get: getter,
          set: setter,
          configurable: descriptor.configurable,
          enumerable: descriptor.enumerable
        });
      }
    }
    function register(nativeConstructor, wrapperConstructor, opt_instance) {