**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 style = rule.style;
        for (var i in style) {
          if (style[i] === "initial") {
            cssText += i + ": initial; ";
          }
        }
        return cssText;
      },
      replaceTextInStyles: function(styles, action) {
        if (styles && action) {
          if (!(styles instanceof Array)) {
            styles = [ styles ];
          }
          Array.prototype.forEach.call(styles, function(s) {
            s.textContent = action.call(this, s.textContent);
          }, this);
        }
      },
      addCssToDocument: function(cssText, name) {
        if (cssText.match("@import")) {
          addOwnSheet(cssText, name);
        } else {
          addCssToDocument(cssText);
        }
      }
    };
    var selectorRe = /([^{]*)({[\s\S]*?})/gim, cssCommentRe = /\/\*[^*]*\*+([^/*][^*]*\*+)*\//gim, cssCommentNextSelectorRe = /\/\*\s*@polyfill ([^*]*\*+([^/*][^*]*\*+)*\/)([^{]*?){/gim, cssContentNextSelectorRe = /polyfill-next-selector[^}]*content\:[\s]*?['"](.*?)['"][;\s]*}([^{]*?){/gim, cssCommentRuleRe = /\/\*\s@polyfill-rule([^*]*\*+([^/*][^*]*\*+)*)\//gim, cssContentRuleRe = /(polyfill-rule)[^}]*(content\:[\s]*['"](.*?)['"])[;\s]*[^}]*}/gim, cssCommentUnscopedRuleRe = /\/\*\s@polyfill-unscoped-rule([^*]*\*+([^/*][^*]*\*+)*)\//gim, cssContentUnscopedRuleRe = /(polyfill-unscoped-rule)[^}]*(content\:[\s]*['"](.*?)['"])[;\s]*[^}]*}/gim, cssPseudoRe = /::(x-[^\s{,(]*)/gim, cssPartRe = /::part\(([^)]*)\)/gim, polyfillHost = "-shadowcsshost", polyfillHostContext = "-shadowcsscontext", parenSuffix = ")(?:\\((" + "(?:\\([^)(]*\\)|[^)(]*)+?" + ")\\))?([^,{]*)";
    var cssColonHostRe = new RegExp("(" + polyfillHost + parenSuffix, "gim"), cssColonHostContextRe = new RegExp("(" + polyfillHostContext + parenSuffix, "gim"), selectorReSuffix = "([>\\s~+[.,{:][\\s\\S]*)?$", colonHostRe = /\:host/gim, colonHostContextRe = /\:host-context/gim, polyfillHostNoCombinator = polyfillHost + "-no-combinator", polyfillHostRe = new RegExp(polyfillHost, "gim"), polyfillHostContextRe = new RegExp(polyfillHostContext, "gim"), shadowDOMSelectorsRe = [ /\^\^/g, /\^/g, /\/shadow\//g, /\/shadow-deep\//g, /::shadow/g, /\/deep\//g, /::content/g ];
    function stylesToCssText(styles, preserveComments) {