Get firstObject/lastObject notifications only when cached

[stefanpenner]

Basically the contents of https://gist.github.com/mmun/877bad33158578a8609b

[fixes #5591]

---

changed:

cacheFor(this, 'firstObject');
cacheFor(this, 'lastObject');

to:

cacheFor.get(cache, 'firstObject');
cacheFor.get(cache, 'lastObject');

this lets us skip needing to re-fetch the cache (we just did above), but not leak the sentinel UNDEFINED value (which is why we have the methods in the first place.

---

• Also switched to objectAt(array, index) and this to array
• Also made length lazy unless lastObject needs accessing

[stefanpenner]

@krisselden / @rwjblue should this ride canary or be a [BUGFIX] of some sort?

[rwjblue]

I think we can go with [BUGFIX beta], we are still super early in the beta cycle...

[stefanpenner]

going to sleep, i'll fix tests and address feedback when I wake up.

[stefanpenner]

@krisselden should we:

• go all the way (like the PR currently does) and compare firstObject to the last firstObject we new about before broadcasting the changes(more aligned with our setter strategy),
• or should we merely broadcast if the cache has been set before (more aligned with our existing CP DK strategy),
• or should we broads if the first index or last index was part of the changeset?

[krisselden]

@stefanpenner it is currently already more like the set() strategy, it was just doing it before firstObject and lastObject were ever consumed which is what this is addressing. I lean toward landing it as is and proposing considering the other changes after

[stefanpenner]

@krisselden sounds good, we can always switch to the more CP style by checking affected array offsets in the future.