Fix a unit division bug

CHANGELOG.md

@@ -16,6 +16,9 @@
 * Fix a bug where multiplication like `(1/1px) * (1px/1)` wouldn't properly
   cancel out units.
 
+* Fix a bug where dividing by a compatible unit would produce an invalid
+  result.
+
 ### Dart API
 
 * Add a `functions` parameter to `compile()`, `compleString()`,

lib/src/value/number.dart

@@ -444,7 +444,7 @@ class SassNumber extends Value implements ext.SassNumber {
       removeFirstWhere<String>(mutableDenominators1, (denominator) {
         var factor = _conversionFactor(numerator, denominator);
         if (factor == null) return false;
-        value *= factor;
+        value /= factor;
         return true;
       }, orElse: () {
         newNumerators.add(numerator);