In strong mode, properly infer the return types of setters as `void`

pkg/front_end/testcases/inference/infer_parameter_type_setter_from_setter.dart.strong.expect

Index: pkg/front_end/testcases/inference/infer_parameter_type_setter_from_setter.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/infer_parameter_type_setter_from_setter.dart.strong.expect b/pkg/front_end/testcases/inference/infer_parameter_type_setter_from_setter.dart.strong.expect
index 82341032bdcdd88604a59f08bb49a17a819ee17d..3d47596ae5f5fff6a042a2c03badbc11101c883d 100644
--- a/pkg/front_end/testcases/inference/infer_parameter_type_setter_from_setter.dart.strong.expect
+++ b/pkg/front_end/testcases/inference/infer_parameter_type_setter_from_setter.dart.strong.expect
@@ -6,12 +6,12 @@ class C extends self::D {
   constructor •() → void
     : super self::D::•()
     ;
-  set foo(core::int x) → dynamic {}
+  set foo(core::int x) → void {}
 }
 class D extends core::Object {
   constructor •() → void
     : super core::Object::•()
     ;
-  set foo(core::int x) → dynamic {}
+  set foo(core::int x) → void {}
 }
 static method main() → dynamic {}