**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

return /^\w[a-zA-Z_0-9]*$/.test(name);
    }
    function getGetter(name) {
      return hasEval && isIdentifierName(name) ? new Function("return this.__impl4cf1e782hg__." + name) : function() {
        return this.__impl4cf1e782hg__[name];
      };
    }
    function getSetter(name) {
      return hasEval && isIdentifierName(name) ? new Function("v", "this.__impl4cf1e782hg__." + name + " = v") : function(v) {
        this.__impl4cf1e782hg__[name] = v;
      };
    }
    function getMethod(name) {
      return hasEval && isIdentifierName(name) ? new Function("return this.__impl4cf1e782hg__." + name + ".apply(this.__impl4cf1e782hg__, arguments)") : function() {
        return this.__impl4cf1e782hg__[name].apply(this.__impl4cf1e782hg__, arguments);
      };
    }
    function getDescriptor(source, name) {
      try {
        return Object.getOwnPropertyDescriptor(source, name);
      } catch (ex) {
        return dummyDescriptor;
      }
    }
    var isBrokenSafari = function() {
      var descr = Object.getOwnPropertyDescriptor(Node.prototype, "nodeType");
      return descr && !descr.get && !descr.set;
    }();
    function installProperty(source, target, allowMethod, opt_blacklist) {