**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 fakeLoad = false;
        if (elt.textContent.indexOf("@import") == -1) {
          fakeLoad = true;
        } else if (elt.sheet) {
          fakeLoad = true;
          var csr = elt.sheet.cssRules;
          var len = csr ? csr.length : 0;
          for (var i = 0, r; i < len && (r = csr[i]); i++) {
            if (r.type === CSSRule.IMPORT_RULE) {
              fakeLoad = fakeLoad && Boolean(r.styleSheet);
            }
          }
        }
        if (fakeLoad) {
          elt.dispatchEvent(new CustomEvent("load", {
            bubbles: false
          }));
        }
      }
    },
    parseScript: function(scriptElt) {
      var script = document.createElement("script");
      script.__importElement = scriptElt;
      script.src = scriptElt.src ? scriptElt.src : generateScriptDataUrl(scriptElt);
      scope.currentScript = scriptElt;
      this.trackElement(script, function(e) {
        script.parentNode.removeChild(script);
        scope.currentScript = null;
      });
      this.addElementToDocument(script);