Fixes #109675.

This formula would produce an initial velocity quite different from the one specified as an argument.

To update the test, I computed the expected results separately by using the physical formula.

Happily, the framework by default never ends up actually exercising this code. Of the four SpringDescription call sites within the framework, two are explicitly overdamped; the other two are by design critically damped, but due to rounding they end up being treated as (very slightly) overdamped too. Details here:
#109675 (comment)

So the only way an app could be affected by this bug is if it called a SpringDescription constructor itself, and managed to create a spring description where the distinguishing formula in _SpringSolution comes out exactly equal to zero. It's likely nobody has ever shipped such an app, because the behavior this produces would be so wildly wrong that it'd be hard to miss when exercised.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.