Fix styles.scss to include alert block styles + test.

pkg/web_css/lib/style.scss

@@ -9,6 +9,7 @@
 // Local styles and rules.
 @use 'src/_variables';
 @use 'src/_base';
+@use 'src/_alerts';
 @use 'src/_site_header';
 @use 'src/_activity_log';
 @use 'src/_detail_page';
@@ -20,6 +21,6 @@
 @use 'src/_report';
 @use 'src/_scores';
 @use 'src/_search';
-@use 'src/_staging_ribbon.scss';
+@use 'src/_staging_ribbon';
 @use 'src/_tags';
 @use 'src/_topics';

pkg/web_css/pubspec.yaml

@@ -10,4 +10,5 @@ dependencies:
 
 dev_dependencies:
   csslib: ^1.0.0
+  path: ^1.9.1
   test: ^1.16.5

pkg/web_css/test/file_use_test.dart

@@ -0,0 +1,33 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:io';
+
+import 'package:path/path.dart' as p;
+import 'package:test/test.dart';
+
+void main() {
+  test('scss files in lib/ are exactly those referenced by style.scss ',
+      () async {
+    final references = (await File('lib/style.scss').readAsLines())
+        .where((l) => l.startsWith(r'@use'))
+        .map((l) =>
+            l.substring(4).trim().split(';').first.replaceAll("'", '').trim())
+        .where((v) => v.isNotEmpty)
+        .toSet();
+    expect(references, isNotEmpty);
+    expect(references, contains('src/_tags'));
+
+    final files = Directory('lib')
+        .listSync(recursive: true)
+        .whereType<File>()
+        .where((f) => f.path.endsWith('.scss'))
+        .map((f) => p.relative(f.path, from: 'lib'))
+        .map((n) => n.substring(0, n.length - 5)) // without the .scss extension
+        .toSet();
+    files.removeAll(['dartdoc', 'style']);
+    expect(files, isNotEmpty);
+    expect(references, files);
+  });
+}