fix(animationFrameScheduler): some tasks are never flushed and sometimes it breaks completely

[pmoleri]

animationFrameScheduler has a few issues that makes it miss tasks and even start accumulating tasks that never get flushed.

• It's clearing the _scheduled in every flush, but some of the flushes are not for the next scheduled frame, they're regular async timeouts.
• because it might be picking the incorrect flushId it may flush tasks before the next frame
• after flushing and recycling the id it's clearing the _scheduled even though current id might be a different one.

Related issues:

• animationFrameScheduler stops working unexpectedly #7018
• animationFrameScheduler with delay causes incorrect behavior (and exception) #6891
• asapScheduler: Scheduling inside of an executing action only works once #7196
  already fixed but similar cause

Failing tests are included in a independent commit and then fixed.

Note:
The changes could also be ported to AsapScheduler for consistency (also I think it's more clear).
However, I couldn't reproduce the issues there because:

• AsapScheduler already contains one of the fixes
• In AsapScheduler I couldn't schedule a task to happen before the _scheduled one.

[pmoleri]

Hi @benlesh. I noticed that you fixed #7196 for asapScheduler, but animationFrameScheduler has similar issue, and worse even (reason explained in this PR description).
Any chance that you can review this PR? It also includes tests that make it fail without the fix.

[BlueCat0]

any update yet?

[pmoleri]

@BlueCat0 Unfortunately it seems that no one had the time to review it.
Once you understand the code (which takes a while) the bug is pretty clear and the resolution too.

To avoid the bug altogether, never use animationFrameScheduler with a delay, you can use the asyncScheduler for that purpose. In fact they would behave exactly the same, as animationFrameScheduler doesn't really schedule into a frame when delay is used.

Note that using animationFrameScheduler with a delay in one place may impact in a different place.