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
rootContainerElement != null &&
      rootContainerElement.nodeType === ELEMENT_NODE,
    'ensureListeningTo(): received a container that was not an element node. ' +
      'This is likely a bug in React.',
  );
  listenToReactEvent(
    reactPropEvent,
    ((rootContainerElement: any): Element),
    targetElement,
  );
}
function getOwnerDocumentFromRootContainer(
  rootContainerElement: Element | Document,
): Document {
  return rootContainerElement.nodeType === DOCUMENT_NODE
    ? (rootContainerElement: any)
    : rootContainerElement.ownerDocument;
}
function noop() {}
export function trapClickOnNonInteractiveElement(node: HTMLElement) {
  
  
  
  
  
  
  
  
  
  node.onclick = noop;