**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 (!registration) {
        registration = new Registration(this, target, options);
        registrations.push(registration);
        this.nodes_.push(target);
      }
      registration.addListeners();
    },
    disconnect: function() {
      this.nodes_.forEach(function(node) {
        var registrations = registrationsTable.get(node);
        for (var i = 0; i < registrations.length; i++) {
          var registration = registrations[i];
          if (registration.observer === this) {
            registration.removeListeners();
            registrations.splice(i, 1);
            break;
          }
        }
      }, this);
      this.records_ = [];
    },
    takeRecords: function() {
      var copyOfRecords = this.records_;
      this.records_ = [];
      return copyOfRecords;
    }
  };
  function MutationRecord(type, target) {