Properly register pseudos' child selectors in ExtensionStore (#1374)

Closes #1297

CHANGELOG.md

@@ -13,6 +13,9 @@
   URLs (those that begin with `/`) in `@import` rules would be passed to
   both Dart and JS importers as `file:` URLs.
 
+* Fix an edge case where `@extend` wouldn't affect a selector within a
+  pseudo-selector such as `:is()` that itself extended other selectors.
+
 ## 1.35.1
 
 * Fix a bug where the quiet dependency flag didn't silence warnings in some

lib/src/extend/extension_store.dart

@@ -258,16 +258,12 @@ class ExtensionStore {
 
       sources[complex] = extension;
 
-      for (var component in complex.components) {
-        if (component is CompoundSelector) {
-          for (var simple in component.components) {
-            _extensionsByExtender.putIfAbsent(simple, () => []).add(extension);
-            // Only source specificity for the original selector is relevant.
-            // Selectors generated by `@extend` don't get new specificity.
-            _sourceSpecificity.putIfAbsent(
-                simple, () => complex.maxSpecificity);
-          }
-        }
+      for (var simple in _simpleSelectors(complex)) {
+        _extensionsByExtender.putIfAbsent(simple, () => []).add(extension);
+        // Only source specificity for the original selector is relevant.
+        // Selectors generated by `@extend` don't get new specificity.
+        _sourceSpecificity.putIfAbsent(
+          simple, () => complex.maxSpecificity);
       }
 
       if (selectors != null || existingExtensions != null) {
@@ -292,6 +288,24 @@ class ExtensionStore {
     }
   }
 
+  /// Returns an iterable of all simple selectors in [complex]
+  Iterable<SimpleSelector> _simpleSelectors(ComplexSelector complex) sync* {
+    for (var component in complex.components) {
+      if (component is CompoundSelector) {
+        for (var simple in component.components) {
+          yield simple;
+
+          if (simple is! PseudoSelector) continue;
+          var selector = simple.selector;
+          if (selector == null) continue;
+          for (var complex in selector.components) {
+            yield* _simpleSelectors(complex);
+          }
+        }
+      }
+    }
+  }
+
   /// Extend [extensions] using [newExtensions].
   ///
   /// Note that this does duplicate some work done by