Re: [w3c/webcomponents] How to avoid memory leaks when detecting custom elements? (#674)

@trusktr I'd like to make sure I understand your need correctly. You would like to give the author a way to explicitly reject all promises for `whenDefined` given custom element? Like,
```js
customElements.define('any-thing', SomeElement);
// SomeElement logic:
for (const elm of Array.from(this.children)) {
    if (elm.nodeName.includes('-')) {
        customElements.whenDefined(elm.nodeName.toLowerCase()).then(() => {
            if (elm instanceof OtherElement) {
               // do something only if the child is an OtherElement
            }
        })
    }
}
// once author knows that `an-element` definition will never come, for example because he/she deliberetly didn't import it
customElements.stopWaitingFor('an-element'); // resolves promise with rejection, letting callback to be collected.
```

or just for a given call? To reject a promise that this particular element will be upgraded.

```js
customElements.define('any-thing', SomeElement);
// SomeElement logic:
for (const elm of Array.from(this.children)) {
    if (elm.nodeName.includes('-')) {
        elm.whenUpgraded.then(() => {
            if (elm instanceof OtherElement) {
               // do something only if the child is an OtherElement
            }
        })
    }
}
// once author knows that given element will never get upgraded to `an-element`, 
rejectUpgradePromise(document.querySelector('any-thing an-element'), 'reason: nuked'); // something that rejects a promise for that instance
```

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