**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

renderer.dirty = false;
        }
      },
      compose: function(node) {
        var children = [];
        var p = node.shadowRoot || node;
        for (var child = p.firstChild; child; child = child.nextSibling) {
          if (isInsertionPoint(child)) {
            this.associateNode(p);
            var distributedNodes = getDistributedNodes(child);
            for (var j = 0; j < distributedNodes.length; j++) {
              var distributedNode = distributedNodes[j];
              if (isFinalDestination(child, distributedNode)) children.push(distributedNode);
            }
          } else {
            children.push(child);
          }
        }
        return children;
      },
      invalidateAttributes: function() {
        this.attributes = Object.create(null);
      },
      updateDependentAttributes: function(selector) {
        if (!selector) return;
        var attributes = this.attributes;
        if (/\.\w+/.test(selector)) attributes["class"] = true;
        if (/#\w+/.test(selector)) attributes["id"] = true;
        selector.replace(/\[\s*([^\s=\|~\]]+)/g, function(_, name) {
          attributes[name] = true;