HTML spec needs further changes in IntersectionObserver for proper integration #464

Follow-up to #427

In whatwg/html#6097 I tried to update the HTML spec to use the new terms, but found additional issues.

• isIntersecting is not referenceable without referring to that API. Specifying it as an internal slot instead would help. Doing this for all things in the spec might be better than playing whack-a-mole.
• Similarly for target. https://w3c.github.io/IntersectionObserver/#dom-intersectionobserverentry-target (There is a term "target", but .target does not use it, and it's not clear if it's a property/internal slot of an IntersectionObserver instance, or a general spec term.)
• HTML initializes a new IntersectionObserver, which requires specifying a callback. Currently, this reads:

The callback is these steps, with arguments entries and observer:

1. For each entry in entries [ using a method of iteration which does not trigger developer-modifiable array accessors or iteration hooks ] :

So... we need a way to enumerate entries, but without triggering developer-modifiable array accessors or iteration hooks. I'm not entirely sure how to solve this concretely, but maybe @annevk or @domenic can advise?

In addition, @annevk mentioned this in IRC: https://w3c.github.io/IntersectionObserver/#initialize-new-intersection-observer says in step 8:

The thresholds attribute getter will return this sorted thresholds list.

This should instead set an internal slot, and the definition of thresholds getter should say to return the value of that internal slot.