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
return;
    }
    didWarnInvalidHydration = true;
    const names = [];
    attributeNames.forEach(function(name) {
      names.push(name);
    });
    console.error('Extra attributes from the server: %s', names);
  };
  warnForInvalidEventListener = function(registrationName, listener) {
    if (listener === false) {
      console.error(
        'Expected `%s` listener to be a function, instead got `false`.\n\n' +
          'If you used to conditionally omit it with %s={condition && value}, ' +
          'pass %s={condition ? value : undefined} instead.',
        registrationName,
        registrationName,
        registrationName,
      );
    } else {
      console.error(
        'Expected `%s` listener to be a function, instead got a value of `%s` type.',
        registrationName,
        typeof listener,
      );
    }
  };
  
  
  normalizeHTML = function(parent: Element, html: string) {