Added support for lerpDuration().

[matthew-carroll]

Adds the ability to linearly interpolate Durations.

[matthew-carroll]

@cbracken @Hixie I have a question on testing for this PR. I did a search through the testing/dart directory in the engine repo and didn't find any existing tests for lerpDouble, which is where I planned to add tests for lerpDuration. Any idea if those tests are located somewhere else, or if we simply don't have any such tests?

[cbracken]

Nice catch. Thanks for spotting this! Just sent #20778 which adds tests for lerpDouble.

[Hixie]

Would it make more sense to put this in the foundation library of the flutter framework? Generally we try to keep dart:ui as small as possible and so we only put things here that we need to have here. (If there's a reason to have it here of course that's fine.)

[matthew-carroll]

@Hixie sure, I can move it. I just went immediately to where lerpDouble was located, but I'll move it to foundation.

[matthew-carroll]

Closing in favor of an implementation in Foundation instead of dart:ui:
flutter/flutter#64668