**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

head.replaceChild(style, elt);
              } else {
                this.addElementToDocument(style);
              }
            }
            style.__importParsed = true;
            this.markParsingComplete(elt);
            this.parseNext();
          };
          var hasResource = HTMLImports.parser.hasResource;
          HTMLImports.parser.hasResource = function(node) {
            if (node.localName === "link" && node.rel === "stylesheet" && node.hasAttribute(SHIM_ATTRIBUTE)) {
              return node.__resource;
            } else {
              return hasResource.call(this, node);
            }
          };
        }
      });
    }
    scope.ShadowCSS = ShadowCSS;
  })(window.WebComponents);
}
(function(scope) {
  if (window.ShadowDOMPolyfill) {
    window.wrap = ShadowDOMPolyfill.wrapIfNeeded;
    window.unwrap = ShadowDOMPolyfill.unwrapIfNeeded;
  } else {
    window.wrap = window.unwrap = function(n) {
      return n;