Rename variable millisecondsPerExercise to microsecondsPerExercise

[moluopro]

Rename variable millisecondsPerExercise to microsecondsPerExercise

This might be an error caused by oversight.

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[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/414480 has been updated with the latest commits from this pull request.

[mraleph]

Thanks!