toSecondResolution needs to handle microseconds #532

The toSecondResolution utility tries to round a DateTime to the nearest whole second by removing milliseconds. However, DateTimes can have microseconds, and as of https://dart.googlesource.com/sdk/+/9f7bf3fcffa32ef89a8c8d4d9d8c4185c1f2324e, will start to have non-zero microseconds when reading file timestamps.

This will come in Dart 3.13. But the fix can be made independently.