validate @mustBeOverridden on extension type methods

See: #53434

Change-Id: Ibeab460fa8cc9d7e7fa0efdb062c839659243e45
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/324574
Reviewed-by: Samuel Rawlins <[email protected]>
Commit-Queue: Phil Quitslund <[email protected]>

Author: pq

pkg/analyzer/lib/src/error/annotation_verifier.dart

@@ -210,6 +210,7 @@ class AnnotationVerifier {
     if ((parent is MethodDeclaration && parent.isStatic) ||
         (parent is FieldDeclaration && parent.isStatic) ||
         parent.parent is ExtensionDeclaration ||
+        parent.parent is ExtensionTypeDeclaration ||
         parent.parent is EnumDeclaration) {
       _errorReporter.reportErrorForNode(
         WarningCode.INVALID_ANNOTATION_TARGET,

pkg/analyzer/test/src/diagnostics/invalid_annotation_target_test.dart

@@ -157,6 +157,19 @@ extension E on String {
     ]);
   }
 
+  test_extensionType_instance_method() async {
+    await assertErrorsInCode(r'''
+import 'package:meta/meta.dart';
+
+extension type E(int i) {
+  @mustBeOverridden
+  void m() { }
+}
+''', [
+      error(WarningCode.INVALID_ANNOTATION_TARGET, 62, 17),
+    ]);
+  }
+
   test_mixin_instance_method() async {
     await assertNoErrorsInCode(r'''
 import 'package:meta/meta.dart';