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
export function warnForDeletedHydratableElement(
  parentNode: Element | Document,
  child: Element,
) {
  if (__DEV__) {
    if (didWarnInvalidHydration) {
      return;
    }
    didWarnInvalidHydration = true;
    console.error(
      'Did not expect server HTML to contain a <%s> in <%s>.',
      child.nodeName.toLowerCase(),
      parentNode.nodeName.toLowerCase(),
    );
  }
}
export function warnForDeletedHydratableText(
  parentNode: Element | Document,
  child: Text,
) {
  if (__DEV__) {
    if (didWarnInvalidHydration) {
      return;
    }
    didWarnInvalidHydration = true;
    console.error(
      'Did not expect server HTML to contain the text node "%s" in <%s>.',
      child.nodeValue,
      parentNode.nodeName.toLowerCase(),
    );