**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

});
      }
      clearRecords();
    }
  };
  global.JsMutationObserver = JsMutationObserver;
  if (!global.MutationObserver) global.MutationObserver = JsMutationObserver;
})(this);
window.HTMLImports = window.HTMLImports || {
  flags: {}
};
(function(scope) {
  var IMPORT_LINK_TYPE = "import";
  var useNative = Boolean(IMPORT_LINK_TYPE in document.createElement("link"));
  var hasShadowDOMPolyfill = Boolean(window.ShadowDOMPolyfill);
  var wrap = function(node) {
    return hasShadowDOMPolyfill ? ShadowDOMPolyfill.wrapIfNeeded(node) : node;
  };
  var rootDocument = wrap(document);
  var currentScriptDescriptor = {
    get: function() {
      var script = HTMLImports.currentScript || document.currentScript || (document.readyState !== "complete" ? document.scripts[document.scripts.length - 1] : null);
      return wrap(script);
    },
    configurable: true
  };
  Object.defineProperty(document, "_currentScript", currentScriptDescriptor);
  Object.defineProperty(rootDocument, "_currentScript", currentScriptDescriptor);
  var isIE = /Trident/.test(navigator.userAgent);
  function whenReady(callback, doc) {