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
normalizedClientText,
    );
  };
  warnForPropDifference = function(
    propName: string,
    serverValue: mixed,
    clientValue: mixed,
  ) {
    if (didWarnInvalidHydration) {
      return;
    }
    const normalizedClientValue = normalizeMarkupForTextOrAttribute(
      clientValue,
    );
    const normalizedServerValue = normalizeMarkupForTextOrAttribute(
      serverValue,
    );
    if (normalizedServerValue === normalizedClientValue) {
      return;
    }
    didWarnInvalidHydration = true;
    console.error(
      'Prop `%s` did not match. Server: %s Client: %s',
      propName,
      JSON.stringify(normalizedServerValue),
      JSON.stringify(normalizedClientValue),
    );
  };
  warnForExtraAttributes = function(attributeNames: Set<string>) {
    if (didWarnInvalidHydration) {