Support FutureOr<T> in the VM (fixes #28010).

runtime/vm/class_finalizer.cc

Index: runtime/vm/class_finalizer.cc
diff --git a/runtime/vm/class_finalizer.cc b/runtime/vm/class_finalizer.cc
index be24a412b4e6cedd580541084fa323ae30e6ec64..c319f1f18c42b75239ae97e4b544a5692e6e2d02 100644
--- a/runtime/vm/class_finalizer.cc
+++ b/runtime/vm/class_finalizer.cc
@@ -516,6 +516,13 @@ void ClassFinalizer::ResolveTypeClass(const Class& cls, const Type& type) {
   }
   ASSERT(!type_class.IsTypedefClass() ||
          (type.signature() != Function::null()));
+
+  // Replace FutureOr<T> type of async library with dynamic.
+  if ((type_class.library() == Library::AsyncLibrary()) &&
+      (type_class.Name() == Symbols::FutureOr().raw())) {
+    Type::Cast(type).set_type_class(Class::Handle(Object::dynamic_class()));
+    type.set_arguments(Object::null_type_arguments());
+  }
 }