Propagate selector source specificity between modules

Closes #866

CHANGELOG.md

@@ -1,3 +1,8 @@
+## 1.23.3
+
+* Fix a bug where selectors were being trimmed over-eagerly when `@extend`
+  crossed module boundaries.
+
 ## 1.23.2
 
 ### Command-Line Interface

lib/src/extend/extender.dart

@@ -400,6 +400,7 @@ class Extender {
 
     for (var extender in extenders) {
       if (extender.isEmpty) continue;
+      _sourceSpecificity.addAll(extender._sourceSpecificity);
       extender._extensions.forEach((target, newSources) {
         // Private selectors can't be extended across module boundaries.
         if (target is PlaceholderSelector && target.isPrivate) return;

pubspec.yaml

@@ -1,5 +1,5 @@
 name: sass
-version: 1.23.2
+version: 1.23.3
 description: A Sass implementation in Dart.
 author: Sass Team
 homepage: https://github.com/sass/dart-sass