Consider re-exporting package:fake_async and package:clock and removing custom impls

[jakemac53]

We currently maintain duplicate implementations of FakeAsync and Clock in the fake_async and clock packages. I haven't done a full evaluation but they seem almost identical.

Can we simply re-export those packages from here instead to keep them in sync?

We could at the same time deprecate the public entrypoints for those libraries and point at those packages.

[cbracken]

Good question -- FakeAsync was originally created here but later broken out into its own package. At one point there definitely were are a few differences but if we haven't yet, we should kill those off. It seems reasonable to either re-export them or simply deprecate the quiver version and redirect users to FakeAsync in the documentation. Will take a look.

[jakemac53]

Note that I have a PR out to fake_async to add the pendingTimersDebugInfo stuff, dart-lang/fake_async#17

[jakemac53]

I realized that this is actually likely a blocker for migrating flutter_test off of quiver - there is at least one test internally which grabs the clock from flutter_test and passes it to something that expects a quiver Clock.

If we just re-export package:clock that solves the problem though.

[jakemac53]

You can pretty easily make a quiver clock from a package:clock clock (quiver.Clock(clock.now)) basically. So not actually a blocker.