Re: [w3c/webcomponents] Element.prototype.connectedCallback, et al. (#629)

This is how my base class implements `childConnectedCallback`/`childDisconnectedCallback `:

```js
// NOTE: If a child is disconnected then connected to the same parent in the
// same turn, then the onConnect and onDisconnect callbacks won't be called
// because the DOM tree will be back in the exact state as before.
function observeChildren(ctx, onConnect, onDisconnect) {

    const observer = new MutationObserver(changes => {
        const weights = new Map

        for (const change of changes) {
            if (change.type != 'childList') continue

            for (const addedNode of change.addedNodes)
                weights.set(addedNode, (weights.get(addedNode) || 0) + 1)

            for (const removedNode of change.removedNodes)
                weights.set(removedNode, (weights.get(removedNode) || 0) - 1)
        }

        for (const [node, weight] of weights)
            if (weight > 0)
                onConnect.call(ctx, node)
            else if (weight < 0)
                onDisconnect.call(ctx, node)
    })

    observer.observe(ctx, { childList: true })
    return observer
}

// ... then inside the base class ...
        observeChildren(this, this.childConnectedCallback, this.childDisconnectedCallback)
```


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