Properly stringify slash-separated numbers in operations (#612)

Closes #607

CHANGELOG.md

@@ -1,5 +1,9 @@
 ## 1.17.3
 
+* Fix an edge case where slash-separated numbers were written to the stylesheet
+  with a slash even when they're used as part of another arithmetic operation,
+  such as being concatenated with a string.
+
 * Don't put style rules inside empty `@keyframes` selectors.
 
 ## 1.17.2

lib/src/parse/stylesheet.dart

@@ -1477,6 +1477,8 @@ relase. For details, see http://bit.ly/moz-document.
             length: operator.operator.length);
       }
 
+      allowSlash = allowSlash && operator == BinaryOperator.dividedBy;
+
       operators ??= [];
       operands ??= [];
       while (operators.isNotEmpty &&