Re: [w3c/webcomponents] Custom Element - untrackable upgrade (#671)

The snippet I use to work around this as a custom element author is:

```js
connectedCallback() {
  ...
  this._upgradeProperty('checked');
}

_upgradeProperty(prop) {
  if (this.hasOwnProperty(prop)) {
    let value = this[prop];
    delete this[prop];
    this[prop] = value;
  }
}
```

[Documented a bit more here](https://developers.google.com/web/fundamentals/web-components/best-practices#lazy-properties) under the heading "Make properties lazy".

I agree this is a tricky gotcha and we ran headlong into it while working on [howto-components](https://github.com/GoogleChromeLabs/howto-components).

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