Returns the time that the Sass file at
url was last modified.
url comes from a call to canonicalize for this importer.
By default, this returns the current time, indicating that
url should be
reloaded on every compilation. If implementations override this to provide
a more accurate time, Sass will be better able to avoid recompiling it
If this throws an exception, the exception is ignored and the current time is used as the modification time.
DateTime modificationTime(Uri url) => DateTime.now();