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

pkg/front_end/testcases/inference/infer_parameter_type_setter_from_setter.dart

 // Copyright (c) 2017, 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.
 
 /*@testedFeatures=inference*/
 library test;
 
 class C extends D {
-  set foo(/*@topType=int*/ x) {}
+  set /*@topType=void*/ foo(/*@topType=int*/ x) {}
 }
 
 class D {
-  set foo(int x) {}
+  set /*@topType=void*/ foo(int x) {}
 }
 
 main() {}