[dart2wasm] Fix exception handling in async functions

Fixes #55347.

Change-Id: I9ced2a4c06e6cb9714dc47e3661f5582c70cdeb0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/361063
Reviewed-by: Erik Ernst <[email protected]>
Reviewed-by: Jackson Gardner <[email protected]>
Commit-Queue: Ömer Ağacan <[email protected]>

Author: osa1

pkg/dart2wasm/lib/async.dart

@@ -1123,8 +1123,11 @@ class AsyncCodeGenerator extends CodeGenerator {
 
     for (int catchIdx = 0; catchIdx < node.catches.length; catchIdx += 1) {
       final Catch catch_ = node.catches[catchIdx];
-      final Catch? nextCatch =
-          node.catches.length < catchIdx ? node.catches[catchIdx + 1] : null;
+
+      final nextCatchIdx = catchIdx + 1;
+      final Catch? nextCatch = nextCatchIdx < node.catches.length
+          ? node.catches[nextCatchIdx]
+          : null;
 
       _emitTargetLabel(innerTargets[catch_]!);
 

tests/language/exception/try_multiple_catch_async.dart

@@ -0,0 +1,52 @@
+// Copyright (c) 2024, 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.
+
+// Tests try-catch in `async`, with multiple `catch`/`on` blocks, where the
+// first type test in the `catch`/`on` blocks fail and the subsequent test
+// passes.
+//
+// This is a regression test for issue #55347.
+
+import 'package:expect/expect.dart';
+
+class MyException implements Exception {
+  MyException([this.message]);
+
+  final String? message;
+
+  @override
+  String toString() => 'MyException($message)';
+}
+
+class MyOtherException implements Exception {
+  MyOtherException([this.message]);
+
+  final String? message;
+
+  @override
+  String toString() => 'MyOtherException($message)';
+}
+
+Future<String> asynchronouslyThrowException() async {
+  throw MyException('Throwing an error!');
+}
+
+Future<String?> test() async {
+  try {
+    await asynchronouslyThrowException();
+    Expect.fail('Exception is not thrown');
+  } on MyOtherException {
+    Expect.fail('Wrong exception caught');
+  } on MyException {
+    return 'Success';
+  } catch (error) {
+    Expect.fail('Wrong exception caught');
+  }
+  Expect.fail('No exception caught');
+  return null;
+}
+
+void main() async {
+  Expect.equals(await test(), 'Success');
+}