Implement support for the relative color syntax of CSS Color 5 (#2098)

Co-authored-by: Natalie Weizenbaum <nweiz@google.com>

Author: stof

CHANGELOG.md

@@ -1,4 +1,8 @@
-## 1.68.1
+## 1.69.0
+
+* Add support for the relative color syntax from CSS Color 5. This syntax
+  cannot be used to create Sass color values. It is always emitted as-is in the
+  CSS output.
 
 ### Dart API
 

lib/src/functions/color.dart

@@ -736,6 +736,11 @@ Object /* SassString | List<Value> */ _parseChannels(
   }
 
   var list = channels.asList;
+  if (list case [SassString(:var text, hasQuotes: false), _, ...]
+      when equalsIgnoreCase(text, "from")) {
+    return _functionString(name, [originalChannels]);
+  }
+
   if (list.length > 3) {
     throw SassScriptException("Only 3 elements allowed, but ${list.length} "
         "were passed.");

pubspec.yaml

@@ -1,5 +1,5 @@
 name: sass
-version: 1.68.1-dev
+version: 1.69.0-dev
 description: A Sass implementation in Dart.
 homepage: https://github.com/sass/dart-sass