**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 (registrations[i] === this) {
            registrations.splice(i, 1);
            break;
          }
        }
      }, this);
    },
    handleEvent: function(e) {
      e.stopImmediatePropagation();
      switch (e.type) {
       case "DOMAttrModified":
        var name = e.attrName;
        var namespace = e.relatedNode.namespaceURI;
        var target = e.target;
        var record = new getRecord("attributes", target);
        record.attributeName = name;
        record.attributeNamespace = namespace;
        var oldValue = e.attrChange === MutationEvent.ADDITION ? null : e.prevValue;
        forEachAncestorAndObserverEnqueueRecord(target, function(options) {
          if (!options.attributes) return;
          if (options.attributeFilter && options.attributeFilter.length && options.attributeFilter.indexOf(name) === -1 && options.attributeFilter.indexOf(namespace) === -1) {
            return;
          }
          if (options.attributeOldValue) return getRecordWithOldValue(oldValue);
          return record;
        });
        break;
       case "DOMCharacterDataModified":
        var target = e.target;
        var record = getRecord("characterData", target);