[CLEANUP beta] Avoid before observers in ArrayProxy

[mmun]

This PR removes one of two remaining internal usages of before observers (one in ArrayProxy and one in EachProxy).

In order to remain API compatible, we simulate arrayWillChange events by caching the values of the ArrayProxy upon accessing (e.g. when calling objectAt or getting the length). After arrayWillChange has been triggered we mark the cache as dirty and fire the arrayDidChange event. This will cause the cache to recompute on next access which will generally occur in response to the arrayDidChange event.

There are two further avenues of work:

1. It should be possible to use the indexes in the _arrangedContentArrayDidChange hook to optimize the syncing of the cache to only the slice that changed.

• It's a bit of tricky code to get this right, but I should probably just bite the bullet so that things like pushObject don't become O(N).

2. It would be a good idea to make observation of arrangedContent lazy. You should only need to observe after you've accessed the value once, otherwise why would you care if it has changed?

• When I tried implementing this it caused quite a few tests to fail in ember and ember-data. From a quick glance, most of these tests were synthetic unit tests and not indicative of how an array proxy would be used in an actual application. In any case, I prefer to make this timing change in a separate PR to make it easier to roll back if necessary.

[mmun]

I implemented an optimization to track the first dirty index and to only update the cache starting at that index. I tried tracking the dirty end index as well but it got messy and I don't think it is worth the effort. The current implementation should be suitable for all real world use cases.

[krisselden]

we shouldn't need to do this until there is something to invalidate