Emit forwarding getter/setter when overriding just a getter or setter.

test/codegen/language/setter_override3_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 super call to access a field in a super class is just a normal
-// field access.
-
+// Tests that a setter in a subclass does not shadow the getter in the
+// superclass.
 import "package:expect/expect.dart";
 
 class A {
-  int y;
+  int _x = 42;
+  int get x => _x;
 }
 
 class B extends A {
-  int get x => super.y;
-  void set x(val) { super.y = val; }
+  void set x(int val) {
+    _x = val;
+  }
 }
 
 void main() {
   var b = new B();
-  b.x = 42;
   Expect.equals(42, b.x);
+
+  b.x = 21;
+  Expect.equals(21, b.x);
 }