[dart2wasm] Fix async* transformation of nested async* functions

With nested `async*` functions, the state of whether we're in an
`async*` function should be restored as the previous state after leaving
the nested function, instead of as `false`.

Fixes #55397.

Change-Id: I5f2a964847ae34e98584f3adad6df7857b81e474
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/361600
Commit-Queue: Ömer Ağacan <omersa@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>

pkg/dart2wasm/lib/transformers.dart

@@ -679,13 +679,14 @@ class _WasmTransformer extends Transformer {
 
   @override
   TreeNode visitFunctionNode(FunctionNode functionNode) {
+    final previousEnclosing = _enclosingIsAsyncStar;
     if (functionNode.dartAsyncMarker == AsyncMarker.AsyncStar) {
       _enclosingIsAsyncStar = true;
       functionNode = _lowerAsyncStar(functionNode) as FunctionNode;
-      _enclosingIsAsyncStar = false;
+      _enclosingIsAsyncStar = previousEnclosing;
       return super.visitFunctionNode(functionNode);
     } else {
-      bool previousEnclosing = _enclosingIsAsyncStar;
+      _enclosingIsAsyncStar = false;
       TreeNode result = super.visitFunctionNode(functionNode);
       _enclosingIsAsyncStar = previousEnclosing;
       return result;

tests/language/async/nested_test.dart

@@ -0,0 +1,31 @@
+// 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.
+
+import "package:async_helper/async_helper.dart";
+import "package:expect/expect.dart";
+
+Stream<String> stream() async* {
+  yield 'a';
+  yield 'b';
+}
+
+Stream<String> expandedStream() async* {
+  final expanded = stream().asyncExpand((s) async* {
+    yield 'before';
+    yield s;
+    yield 'after';
+  });
+
+  yield* expanded;
+}
+
+test() async {
+  Expect.listEquals(['before', 'a', 'after', 'before', 'b', 'after'],
+      await expandedStream().toList());
+}
+
+void main() {
+  asyncStart();
+  test().then((_) => asyncEnd());
+}