@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?

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

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.)

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

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