**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 baseTag = definition.extends;
    for (var i = 0, a; a = definition.ancestry[i]; i++) {
      baseTag = a.is && a.tag;
    }
    definition.tag = baseTag || definition.__name;
    if (baseTag) {
      definition.is = definition.__name;
    }
  }
  function resolvePrototypeChain(definition) {
    if (!Object.__proto__) {
      var nativePrototype = HTMLElement.prototype;
      if (definition.is) {
        var inst = document.createElement(definition.tag);
        var expectedPrototype = Object.getPrototypeOf(inst);
        if (expectedPrototype === definition.prototype) {
          nativePrototype = expectedPrototype;
        }
      }
      var proto = definition.prototype, ancestor;
      while (proto && proto !== nativePrototype) {
        ancestor = Object.getPrototypeOf(proto);
        proto.__proto__ = ancestor;
        proto = ancestor;
      }
      definition.native = nativePrototype;
    }
  }
  function instantiate(definition) {