MutationObserver shadow-dom show NotFoundError in IE11

### Description

See the example.

### Example

```html

shadowHost.attachShadow({mode:'open'}).innerHTML = '<div></div>';

(new MutationObserver(function (mutationsList) {
console.info('mutated');
})).observe(shadowHost.shadowRoot, { childList: true });

shadowHost.shadowRoot.appendChild(document.createElement('div'));

```

### Steps to reproduce

see the example

#### Expected behavior

No Error

#### Actual behavior

`NotFoundError`

### Version

@webcomponents/webcomponentsjs@2.6.0

### Browsers affected

- [ ] Chrome
- [ ] Firefox
- [ ] Edge
- [ ] Safari
- [x] IE 11