[dart2wasm] Fix async* stream types when fn return type is not Stream

Fixes #53234.

Change-Id: I97aec5d61bf442ecdaeb2ea2dc61d499ab167dd2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/340862
Reviewed-by: Martin Kustermann <[email protected]>
Commit-Queue: Ömer Ağacan <[email protected]>

Author: osa1

pkg/dart2wasm/lib/transformers.dart

@@ -601,23 +601,28 @@ class _WasmTransformer extends Transformer {
         callAsyncMap, Name('where'), Arguments([whereFilter]),
         interfaceTarget: whereProc, functionType: whereProcType);
 
-    // Finally call cast
-    DartType typeArgument;
-    if (functionNode.returnType is InterfaceType) {
-      typeArgument =
-          (functionNode.returnType as InterfaceType).typeArguments.single;
+    // Finally call cast.
+
+    // Stream element type is defined in language spec section 9. If the return
+    // type is `Stream<U>` then the element type is `U`. Otherwise it needs to
+    // be a supertype of `Object` and the element type is `dynamic`.
+    final DartType streamTypeArgument;
+    final DartType functionReturnType = functionNode.returnType;
+    if (functionReturnType is InterfaceType &&
+        functionReturnType.classNode == coreTypes.streamClass) {
+      streamTypeArgument = functionReturnType.typeArguments.single;
     } else {
-      typeArgument = const DynamicType();
+      streamTypeArgument = const DynamicType();
     }
     Procedure castProc =
         coreTypes.index.getProcedure('dart:async', 'Stream', 'cast');
-    final returnStreamType = InterfaceType(
-        coreTypes.streamClass, typeArgument.nullability, [typeArgument]);
+    final returnStreamType = InterfaceType(coreTypes.streamClass,
+        streamTypeArgument.nullability, [streamTypeArgument]);
     final castProcType = FunctionType(
         [], returnStreamType, Nullability.nonNullable,
         requiredParameterCount: 1);
     final castToExpectedType = InstanceInvocation(InstanceAccessKind.Instance,
-        callWhere, Name('cast'), Arguments([], types: [typeArgument]),
+        callWhere, Name('cast'), Arguments([], types: [streamTypeArgument]),
         interfaceTarget: castProc, functionType: castProcType);
     return FunctionNode(
         Block([