[dart2wasm] Fix a bug in async* desugaring

The desugared code uses `Completer<bool>` values to suspend the `async*`
function, but we can't use a type test and to check if a value is the
`Completer<bool>` value for the suspension or a user-emitted value as
the `async*` function can also yield `Completer<bool>` values. Update
the test from `value is Completer<bool>` to `!isEven`.

Change-Id: I74f54b838e6a2aab942154ec7f3e667e291523e0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/304880
Reviewed-by: Joshua Litt <joshualitt@google.com>
Commit-Queue: Ömer Ağacan <omersa@google.com>

pkg/dart2wasm/lib/transformers.dart

@@ -522,7 +522,7 @@ class _WasmTransformer extends Transformer {
             ]),
             null),
         IfStatement(
-            IsExpression(VariableGet(completerPrePassArg), completerBoolType),
+            Not(VariableGet(isEven)),
             ExpressionStatement(InstanceInvocation(
                 InstanceAccessKind.Instance,
                 VariableGet(completerPrePassArg),

pkg/front_end/testcases/dart2wasm/yield.dart.strong.transformed.expect

@@ -101,7 +101,7 @@ static method asyncMethod(asy::Stream<core::int> stream) → asy::Stream<core::i
       #body(){() → asy::Future<void>};
       return #C3;
     }
-    if(value is asy::Completer<core::bool>)
+    if(!#isEven)
       value.{asy::Completer::complete}(#C2){([FutureOr<core::bool>?]) → void};
     return value;
   }){((core::Object?) → FutureOr<core::Object?>) → asy::Stream<core::Object?>}.{asy::Stream::where}((synthesized core::Object? value) → core::Object? {

tests/web/wasm/asyncstar_desugaring_test.dart

@@ -0,0 +1,25 @@
+// 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 "dart:async";
+
+import 'package:expect/expect.dart';
+
+// async* desugaring uses `Completer<bool>` values to suspend the async*
+// function until the last emitted value is consumed. Check that the desugared
+// code distinguishes user-written `Completer<bool>` values from the values
+// used by the desugared code.
+Stream<Completer<bool>> test() async* {
+  yield Completer<bool>();
+  yield Completer<bool>();
+  yield Completer<bool>();
+}
+
+void main() async {
+  final values = await test().toList();
+  Expect.equals(values.length, 3);
+  for (final completer in values) {
+    Expect.isFalse(completer.isCompleted);
+  }
+}