[analysis_server] Fix location of ignore comments on getters with dartdocs

Fixes https://github.com/Dart-Code/Dart-Code/issues/4775

Change-Id: I7fd049f2f58ed677c5c4a8e5ab7450e54ae61c9c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/329740
Reviewed-by: Phil Quitslund <pquitslund@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

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

@@ -97,8 +97,9 @@ class IgnoreDiagnosticOnLine extends AbstractIgnoreDiagnostic {
   Future<void> compute(ChangeBuilder builder) async {
     if (_isCodeUnignorable()) return;
 
+    final diagnostic = this.diagnostic!; // Enforced by _isCodeUnignorable
     final insertDesc = CorrectionUtils_InsertDesc();
-    insertDesc.offset = node.offset;
+    insertDesc.offset = diagnostic.problemMessage.offset;
     await _computeEdit(
       builder,
       insertDesc,

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

@@ -148,6 +148,18 @@ void f() {
 ''');
   }
 
+  Future<void> test_dartdoc_getter() async {
+    await resolveTestCode('''
+/// AA
+String get _AA => '';
+''');
+    await assertHasFix('''
+/// AA
+// ignore: unused_element
+String get _AA => '';
+''');
+  }
+
   Future<void> test_existingIgnore() async {
     await resolveTestCode('''
 void f() {