[dart2wasm] Fix sync* iterable element types

Fixes tests:

- co19/Language/Functions/element_type_A01_t03
- co19/Language/Functions/element_type_A01_t04
- co19/Language/Functions/element_type_A01_t06

Fixes #54413.

Same change for `async*` functions was made in
https://dart-review.googlesource.com/c/sdk/+/342561.

Change-Id: Ib9e23b3fff9e2f1b952b5ba114ec70d5a8aea372
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/343141
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Ömer Ağacan <omersa@google.com>

pkg/dart2wasm/lib/sync_star.dart

@@ -264,11 +264,7 @@ class SyncStarCodeGenerator extends CodeGenerator {
       FunctionNode functionNode, Context? context, w.BaseFunction resumeFun) {
     // Instantiate a [_SyncStarIterable] containing the context and resume
     // function for this `sync*` function.
-    DartType returnType = functionNode.returnType;
-    DartType elementType = returnType is InterfaceType &&
-            returnType.classNode == translator.coreTypes.iterableClass
-        ? returnType.typeArguments.single
-        : DynamicType();
+    DartType elementType = functionNode.emittedValueType!;
     translator.functions.allocateClass(syncStarIterableInfo.classId);
     b.i32_const(syncStarIterableInfo.classId);
     b.i32_const(initialIdentityHash);