**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 anyDistributed = false;
          for (var i = 0; i < pool.length; i++) {
            var node = pool[i];
            if (!node) continue;
            if (matches(node, content)) {
              destributeNodeInto(node, content);
              pool[i] = undefined;
              anyDistributed = true;
            }
          }
          if (!anyDistributed) {
            for (var child = content.firstChild; child; child = child.nextSibling) {
              destributeNodeInto(child, content);
            }
          }
          return;
        }
        for (var child = node.firstChild; child; child = child.nextSibling) {
          this.poolDistribution(child, pool);
        }
      },
      buildRenderTree: function(renderNode, node) {
        var children = this.compose(node);
        for (var i = 0; i < children.length; i++) {
          var child = children[i];
          var childRenderNode = renderNode.append(child);
          this.buildRenderTree(childRenderNode, child);
        }
        if (isShadowHost(node)) {
          var renderer = getRendererForHost(node);