Implement override-based inference of instance fields.

pkg/front_end/testcases/inference/infer_field_static.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;
+
+dynamic f() => null;
+
+abstract class A {
+  static int get x => 0;
+}
+
+// Even though B extends A, A.x and B.x are unrelated because they're static.
+// So B.x doesn't inherit A.x's type.
+
+class B extends A {
+  static var /*@topType=dynamic*/ x = f();
+}
+
+// Similar with C.x.  It is not even eligible for inference since it's static
+// and has no initializer.
+
+class C extends A {
+  static var x;
+}
+
+main() {}