Use correct notation for multiple denominator units (#2375)

See: https://physics.nist.gov/cuu/Units/checklist.html

* use parentheses

CHANGELOG.md

@@ -13,6 +13,9 @@
 * Fix a slight inaccuracy case when converting to `srgb-linear` and
   `display-p3`.
 
+* **Potentially breaking bug fix:** `math.unit()` now wraps multiple denominator
+  units in parentheses. For example, `px/(em*em)` instead of `px/em*em`.
+
 ### JS API
 
 * Fix `SassColor.interpolate()` to allow an undefined `options` parameter, as

lib/src/value/number.dart

@@ -852,7 +852,8 @@ abstract class SassNumber extends Value {
         ([], [var denominator]) => "$denominator^-1",
         ([], _) => "(${denominators.join('*')})^-1",
         (_, []) => numerators.join("*"),
-        _ => "${numerators.join("*")}/${denominators.join("*")}"
+        (_, [var denominator]) => "${numerators.join("*")}/$denominator",
+        _ => "${numerators.join("*")}/(${denominators.join("*")})",
       };
 
   bool operator ==(Object other) {