Avoid the /-as-division warning when running the dart_api tests

[stof]

Without this, the test output is full of deprecation warnings looking like that:

00:04 +314: test/dart_api/value/number_test.dart: an integer with numerator and denominator units doesn't equal a number with different units
DEPRECATION WARNING: Using / for division is deprecated and will be removed in Dart Sass 2.0.0.

Recommendation: math.div(123px, 5ms)

More info and automated migrator: https://sass-lang.com/d/slash-div

  ╷
3 │     a {b: foo((123px / 5ms))}
  │                ^^^^^^^^^^^
  ╵
    - 3:16  root stylesheet