Implement type inference of getters/setters based on inheritance.

pkg/front_end/testcases/inference/infer_setter_from_later_inferred_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 A implements B {
+  void set x(/*@topType=int*/ value) {}
 }
 
-class D {
-  int foo;
+abstract class B implements C {
+  void set x(/*@topType=int*/ value);
+}
+
+abstract class C {
+  void set x(int value);
 }
 
 main() {}