[w3c/webcomponents] Constructing a custom element synchronously drops prefix from its name (#607)

Namely, 
```js
let x = document.createElementNS('http://www.w3.org/1999/xhtml', 'h:div')
```
then `x.prefix` is `"h"` but

```js
class Custom extends HTMLElement {}
let y = document.createElementNS('http://www.w3.org/1999/xhtml', 'my-custom')
```
then then `y.prefix` is `null`.

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