React version: 0.0.0-f77c7b9d7

## Steps To Reproduce

1. Mount a component with even listeners in a `ShadowRoot`

Link to code example:

- [codesandbox with `react@next`](https://codesandbox.io/s/next-shadowroot-as-root-element-pkktc) (crashes)
- [codesandbox with `react@16.13.1`](https://codesandbox.io/s/16-shadowroot-as-root-element-m74ob) (mounts  without errors/warnings)


## The current behavior

Throws with 
> ensureListeningTo(): received a container that was not an element node. This is likely a bug in React.

## The expected behavior

1. throw with a descriptive error that ShadowRoot is no longer supported as a root element

or

2. continue to allow ShadowRoot as a root element

No preference here. This was just from an old regression test we had. 

Will be fixed by https://github.com/facebook/react/pull/15894
const testElement =
      parent.namespaceURI === HTML_NAMESPACE
        ? parent.ownerDocument.createElement(parent.tagName)
        : parent.ownerDocument.createElementNS(
            (parent.namespaceURI: any),
            parent.tagName,
          );
    testElement.innerHTML = html;
    return testElement.innerHTML;
  };
}
export function ensureListeningTo(
  rootContainerInstance: Element | Node,
  reactPropEvent: string,
  targetElement: Element | null,
): void {
  
  
  const rootContainerElement =
    rootContainerInstance.nodeType === COMMENT_NODE
      ? rootContainerInstance.parentNode
      : rootContainerInstance;
  
  
  
  invariant(