Implement override-based inference of instance fields.

pkg/front_end/testcases/inference/infer_field_overrides_getter.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;
+
+abstract class A {
+  int get x;
+}
+
+class B {
+  int get x => 0;
+}
+
+class C extends A {
+  var /*@topType=int*/ x;
+}
+
+class D extends B {
+  var /*@topType=int*/ x;
+}
+
+class E implements A {
+  var /*@topType=int*/ x;
+}
+
+class F implements B {
+  var /*@topType=int*/ x;
+}
+
+class G extends Object with B {
+  var /*@topType=int*/ x;
+}
+
+main() {}