Use lazy static fields when overriding static getters or setters.

test/codegen/language/static_field_override1_test.dart

 // Copyright (c) 2016, 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.
 
-import "package:expect/expect.dart";
+import 'package:expect/expect.dart';
+
+class Foo {
+  static int get x => 42;
+}
+
+class Bar extends Foo {
+  static int x = 12;
+}
 
 void main() {
-  dynamic x = 3;
-  Expect.equals(3, x.hashCode);
-}
+  Expect.equals(12, Bar.x);
+}