[Custom Elements] Error in IE11: The custom element being constructed was not registered with `customElements`.

### Description

`CustomElementInternals.js` causes an error in IE.

#### Live Demo

Run this pen in IE11:

https://codepen.io/anon/pen/dJerjB

#### Steps to Reproduce

See the pen in IE11. You can use http://crossbrowsertesting.com to test the pen in IE11 for free (click "Change View" in the pen to see a link to "Open on CrossBrowserTesting".

#### Expected Results

no error

#### Actual Results

You'll get an error

```
SCRIPT5022: The custom element being constructed was not registered with `customElements`.
```

pointing to this line:

https://github.com/webcomponents/custom-elements/blob/134c58e6b21a6788f18b715ac5d70ede0cad779f/src/CustomElementInternals.js#L275

(on column `7`, and based on the source map)

### Browsers Affected

- [ ] Chrome
- [ ] Firefox
- [ ] Edge
- [ ] Safari 9
- [ ] Safari 8
- [x] IE 11

### Versions
- `webcomponentsjs@1.0.22`, using https://unpkg.com/@webcomponents/webcomponentsjs@1.0.22/webcomponents-sd-ce.js