Fix rounding when handling negative timestamps

[sigurdm]

Fixes: #577

[osa1]

This looks good to me, thanks.

I'm new to Dart so it took a while for me to get this. "Microseconds since epoch" value is a 64-bit signed integer. The line changed in this PR:

target.seconds = Int64(micros ~/ Duration.microsecondsPerSecond);

truncates a negative number in some cases and effectively does ceiling division when micros is negative. In the original repro, microsecondsSinceEpoch value is -1262393999999000. This program demonstrates the issue:

void main() {
  print(-1262393999999000 / 1000000);           // -1262393999.999
  print(-1262393999999000 ~/ 1000000);          // -1262393999
  print((-1262393999999000 / 1000000).floor()); // -1262394000
}