Remove virtualField helper, just emit getters/setters in codegen.

test/codegen/language/final_field_override_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.
 
-// Tests that a setter in a subclass does not shadow the getter in the
-// superclass.
 import "package:expect/expect.dart";
 
 class A {
   int _x = 42;
+  void set x(int val) {
+    _x = val;
+  }
   int get x => _x;
 }
 
 class B extends A {
-  void set x(int val) {
-    _x = val;
-  }
+  final x = 3;
+  // we can still get to the super property
+  int get y => _x;
 }
 
 void main() {
   var b = new B();
-  Expect.equals(42, b.x);
+  Expect.equals(3, b.x);
 
   b.x = 21;
-  Expect.equals(21, b.x);
+  Expect.equals(3, b.x);
+  Expect.equals(21, b.y);
 }