[dart2js] Fix #51558 - skip abstract getters

Improve 'enum switch' optimization to not be confused by abstract
getters.

Bug: 51558
Change-Id: Iac4e5238c1884df6afd1611aecc9f158b1a8b167
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/286102
Reviewed-by: Nate Biggs <natebiggs@google.com>
Commit-Queue: Stephen Adams <sra@google.com>

pkg/compiler/lib/src/ssa/optimize.dart

@@ -1209,10 +1209,21 @@ class SsaInstructionSimplifier extends HBaseVisitor<HInstruction>
   }
 
   FieldEntity _indexFieldOfEnumClass(ClassEntity enumClass) {
-    final member = _closedWorld.elementEnvironment
+    // We expect the enum class to extend `_Enum`, which has an `index` field,
-        .lookupClassMember(enumClass, const PublicName('index'));
+    // but that might be shadowed by an abstract getter from the class or a
-    if (member is FieldEntity) return member;
+    // mixin.
-    throw StateError('$enumClass should have index field, found $member');
+    ClassEntity? cls = enumClass;
+    MemberEntity? foundMember;
+    while (cls != null) {
+      final member = _closedWorld.elementEnvironment
+          .lookupClassMember(cls, const PublicName('index'));
+      if (member == null) break; // should never happen.
+      foundMember = member;
+      if (member is FieldEntity) return member;
+      if (!member.isAbstract) break;
+      cls = _closedWorld.elementEnvironment.getSuperClass(cls);
+    }
+    throw StateError('$enumClass should have index field, found $foundMember');
   }
 
   IntConstantValue _indexValueOfEnumConstant(

tests/web/regress/issue/51558_test.dart

@@ -0,0 +1,36 @@
+// Copyright (c) 2023, 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 'package:expect/expect.dart';
+
+// Regression test for http://dartbug.com/51558
+//
+// Enum classes may have abstract getters in the inheritance chain that, as
+// declarations, shadow the `index` of the base class.
+
+abstract class EnumFlag extends Object {
+  int get index;
+
+  int get value => 1 << index;
+}
+
+enum EnumXY with EnumFlag { x, y }
+
+extension EnumXTExtensions on EnumXY {
+  String display() {
+    switch (this) {
+      case EnumXY.x:
+        return 'X';
+      case EnumXY.y:
+        return 'Y';
+    }
+  }
+}
+
+void main() {
+  Expect.equals('X', EnumXY.x.display());
+
+  // This second line is needed so that the abstract getter is resolved.
+  Expect.equals(0, EnumXY.x.index);
+}