Add timestamp to AnimationFrame

[bbugh]

This adds the expected Number argument from requestAnimationFrame so that consumers can calculate the time step difference, etc.

Fixes #445

[gdotdesign]

Please update the tests, otherwise it looks good 👍

[bbugh]

Ah, I forgot to mention that. Hard to think while holding a squirmy child...

I wasn't sure how to test the timestamps, given that the argument is non-deterministic. In other tools, I'd use a mock or stub tool, or something like timecop to guarantee consistent results.

I looked through the source for some way to mock it and/or make a strict timestamp result, and I couldn't find anything. Do you have a suggestion?

[Sija]

@bbugh I'd change the tests to assert that the passed argument is greater than 0. That should be good enough.

[bbugh]

There does not seem to be an assertion for greater than, and I don't see one for not equal, or any negation at all. I will make another issue for this.

[bbugh]

Added discussion #458

[bbugh]

@gdotdesign thanks for the updates! It's useful to see how you would write those tests, as the resident Mint expert. ;)

@Sija in case you didn't notice, the changes you've requested were put in by @gdotdesign's commits, so I'm unsure if I should accept the suggestions.

[Sija]

@bbugh I haven't, sorry :D I still believe though we should split those.