Fix extension type import library

Closes #56781

Fixes #55443

GitOrigin-RevId: b6d7b3d
Change-Id: I708da5d01ccae45cc7d143e4fac365fb2c84e31e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/386701
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>

pkg/analysis_server/lib/src/services/correction/dart/import_library.dart

@@ -380,6 +380,7 @@ class ImportLibrary extends MultiCorrectionProducer {
       return await _importLibraryForElement(typeName, const [
         ElementKind.CLASS,
         ElementKind.ENUM,
+        ElementKind.EXTENSION_TYPE,
         ElementKind.FUNCTION_TYPE_ALIAS,
         ElementKind.MIXIN,
         ElementKind.TYPE_ALIAS,

pkg/analysis_server/test/src/services/correction/fix/import_library_project_test.dart

@@ -305,6 +305,22 @@ void f() {
 ''');
   }
 
+  Future<void> test_extensionType_name_notImported() async {
+    newFile('$testPackageLibPath/lib.dart', '''
+extension type ET(String it) {}
+''');
+    await resolveTestCode('''
+void f(ET s) {
+}
+''');
+    await assertHasFix('''
+import 'package:test/lib.dart';
+
+void f(ET s) {
+}
+''');
+  }
+
   Future<void> test_extensionType_notImported() async {
     newFile('$testPackageLibPath/lib.dart', '''
 extension type ET(String it) {}