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
getHostProps as ReactDOMTextareaGetHostProps,
  postMountWrapper as ReactDOMTextareaPostMountWrapper,
  updateWrapper as ReactDOMTextareaUpdateWrapper,
  restoreControlledState as ReactDOMTextareaRestoreControlledState,
} from './ReactDOMTextarea';
import {track} from './inputValueTracking';
import setInnerHTML from './setInnerHTML';
import setTextContent from './setTextContent';
import {
  createDangerousStringForStyles,
  setValueForStyles,
  validateShorthandPropertyCollisionInDev,
} from '../shared/CSSPropertyOperations';
import {Namespaces, getIntrinsicNamespace} from '../shared/DOMNamespaces';
import {
  getPropertyInfo,
  shouldIgnoreAttribute,
  shouldRemoveAttribute,
} from '../shared/DOMProperty';
import assertValidProps from '../shared/assertValidProps';
import {
  DOCUMENT_NODE,
  ELEMENT_NODE,
  COMMENT_NODE,
} from '../shared/HTMLNodeType';
import isCustomComponent from '../shared/isCustomComponent';
import possibleStandardNames from '../shared/possibleStandardNames';
import {validateProperties as validateARIAProperties} from '../shared/ReactDOMInvalidARIAHook';
import {validateProperties as validateInputProperties} from '../shared/ReactDOMNullInputValuePropHook';
import {validateProperties as validateUnknownProperties} from '../shared/ReactDOMUnknownPropertyHook';