Implement override-based inference of instance fields.

pkg/front_end/testcases/inference/infer_field_override_getter_overrides_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 {
+abstract class A {
+  void set x(num value);
+}
+
+abstract class B extends A {
+  int get x;
+}
+
+// The getter in B doesn't screen the setter in A, so inference sees two
+// different types and gives an error.
+class C extends B {
   var /*@topType=dynamic*/ x;
 }
 
-class B implements A {
-  var /*@topType=dynamic*/ x = 2;
-}
-
-foo() {
-  String y = /*info:DYNAMIC_CAST*/ new B(). /*@target=B::x*/ x;
-  int z = /*info:DYNAMIC_CAST*/ new B(). /*@target=B::x*/ x;
-}
+main() {}