Implement override-based inference of instance fields.

pkg/front_end/testcases/inference/infer_final_field_getter_and_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 A {
-  var /*@topType=dynamic*/ x;
+abstract class A {
+  int get x;
+  void set x(double value);
 }
 
-class B implements A {
-  var /*@topType=dynamic*/ x = 2;
+class B extends A {
+  final /*@topType=int*/ x;
+
+  B(this.x);
 }
 
-foo() {
-  String y = /*info:DYNAMIC_CAST*/ new B(). /*@target=B::x*/ x;
-  int z = /*info:DYNAMIC_CAST*/ new B(). /*@target=B::x*/ x;
-}
+main() {}