Fix a couple bugs with @extend

We weren't properly merging multiple @extend rules that were added to
an upstream module at the same time, so some of them weren't being
marked as satisfied.

Closes #1393

CHANGELOG.md

@@ -7,6 +7,9 @@
 
 * Properly parse backslash escapes within `url()` expressions.
 
+* Fix a couple bugs where `@extend`s could be marked as unsatisfied when
+  multiple identical `@extend`s extended selectors across `@use` rules.
+
 ### Command Line Interface
 
 * Strip CRLF newlines from snippets of the original stylesheet that are included

lib/src/extend/extension_store.dart

@@ -409,7 +409,7 @@ class ExtensionStore {
   /// Extends [this] with all the extensions in [extensions].
   ///
   /// These extensions will extend all selectors already in [this], but they
-  /// will *not* extend other extensions from [extenders].
+  /// will *not* extend other extensions from [extensionStores].
   void addExtensions(Iterable<ExtensionStore> extensionStores) {
     // Extensions already in [this] whose extenders are extended by
     // [extensions], and thus which need to be updated.
@@ -445,21 +445,18 @@ class ExtensionStore {
         // Add [newSources] to [_extensions].
         var existingSources = _extensions[target];
         if (existingSources == null) {
-          _extensions[target] = newSources;
+          _extensions[target] = Map.of(newSources);
           if (extensionsForTarget != null || selectorsForTarget != null) {
-            (newExtensions ??= {})[target] = newSources;
+            (newExtensions ??= {})[target] = Map.of(newSources);
           }
         } else {
           newSources.forEach((extender, extension) {
-            // If [extender] already extends [target] in [_extensions], we don't
-            // need to re-run the extension.
-            if (existingSources.containsKey(extender)) return;
-            existingSources[extender] = extension;
+            extension = existingSources.putOrMerge(
+                extender, extension, MergedExtension.merge);
 
             if (extensionsForTarget != null || selectorsForTarget != null) {
-              (newExtensions ??= {})
-                  .putIfAbsent(target, () => {})
-                  .putIfAbsent(extender, () => extension);
+              (newExtensions ??= {}).putIfAbsent(target, () => {})[extender] =
+                  extension;
             }
           });
         }

lib/src/utils.dart

@@ -403,3 +403,15 @@ extension SpanExtensions on FileSpan {
         : file.span(this.start.offset + start, this.start.offset + end + 1);
   }
 }
+
+extension MapExtension<K, V> on Map<K, V> {
+  /// If [this] doesn't contain the given [key], sets that key to [value] and
+  /// returns it.
+  ///
+  /// Otherwise, calls [merge] with the existing value and [value] and sets
+  /// [key] to the result.
+  V putOrMerge(K key, V value, V Function(V oldValue, V newValue) merge) =>
+      containsKey(key)
+          ? this[key] = merge(this[key]!, value)
+          : this[key] = value;
+}