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 wasCustomComponentTag = isCustomComponent(tag, lastRawProps);
  const isCustomComponentTag = isCustomComponent(tag, nextRawProps);
  
  updateDOMProperties(
    domElement,
    updatePayload,
    wasCustomComponentTag,
    isCustomComponentTag,
  );
  
  
  switch (tag) {
    case 'input':
      
      
      
      ReactDOMInputUpdateWrapper(domElement, nextRawProps);
      break;
    case 'textarea':
      ReactDOMTextareaUpdateWrapper(domElement, nextRawProps);
      break;
    case 'select':
      
      
      ReactDOMSelectPostUpdateWrapper(domElement, nextRawProps);
      break;
  }
}
function getPossibleStandardName(propName: string): string | null {
  if (__DEV__) {