Implement type inference of getters/setters based on inheritance.

pkg/front_end/testcases/inference/non_inferrable_getter_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(x) {}
-}
-
-class D {
-  int foo;
+class C {
+  get /*@topType=dynamic*/ x => null;
+  void set x(/*@topType=dynamic*/ value) {}
 }
 
 main() {}