[ddc] Handle optional/defaulted parameters when created scoped parameter renames for sync* transform.

This was missed in the original implementation of the sync* transformer because prior to my recent change, ScopedIds couldn't end up within a DestructuredVariable.

Change-Id: I2733ce1e01edb50659634347204bac1769269615
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/401080
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Commit-Queue: Nate Biggs <natebiggs@google.com>

pkg/dev_compiler/lib/src/compiler/rewrite_async.dart

@@ -2197,8 +2197,13 @@ class SyncStarRewriter extends AsyncRewriterBase {
     for (var parameter in parameters) {
       final name = parameter.parameterName;
       final renamedIdentifier = ScopedId(name);
-      final parameterRef =
-          parameter is ScopedId ? parameter : js_ast.Identifier(name);
+      final parameterRef = switch (parameter) {
+        ScopedId() => ScopedId.from(parameter),
+        js_ast.DestructuredVariable() when parameter.name is ScopedId =>
+          ScopedId.from(parameter.name as ScopedId),
+        _ => js_ast.Identifier(name)
+      };
+
       innerDeclarationsList
           .add(js_ast.VariableInitialization(parameterRef, renamedIdentifier));
       outerDeclarationsList

tests/web/sync_star_optional_parameter_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.
+
+// Ensure optional parameters get renamed properly for sync* transformations.
+
+Iterable<num> range(num startOrStop, [num? stop, num? step]) sync* {
+  final start = stop == null ? 0 : startOrStop;
+  stop ??= startOrStop;
+  step ??= 1;
+
+  if (step == 0) throw ArgumentError('step cannot be 0');
+  if (step > 0 && stop < start) {
+    throw ArgumentError('if step is positive, stop must be greater than start');
+  }
+  if (step < 0 && stop > start) {
+    throw ArgumentError('if step is negative, stop must be less than start');
+  }
+
+  for (
+    num value = start;
+    step < 0 ? value > stop : value < stop;
+    value += step
+  ) {
+    yield value;
+  }
+}
+
+void main() {
+  print(range(10, 20, 2));
+}