Fix field-accesses for private fields that were "shadowed" by other private fields.

tests/language/private1.dart

 // Copyright (c) 2012, 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.
 
 // Dart test for testing access to private fields.
 
 main() {
   testPrivateTopLevel();
   testPrivateClasses();
 }
@@ skipping 62 matching lines @@
 
   var a2 = new AImported();
   Expect.equals(499, a2.getFieldA());
 
   B b = new B();
   Expect.equals(42, b._fieldB);
   Expect.equals(42, accessFieldB2(b));
   expectCatch(() => LibOther3.accessFieldB3(b));
 
   C4 c = new C4();
+  Expect.equals(499, c._field1);
   Expect.equals(499, c_field1a(c));
   Expect.equals(499, c.field1a());
+  Expect.equals(42, c._field2);
   Expect.equals(42, c_field2a(c));
   Expect.equals(42, c.field2a());
   Expect.equals(99, LibOther3.c_field1b(c));
   Expect.equals(99, c.field1b());
   Expect.equals(1024, LibOther3.c_field2b(c));
   Expect.equals(1024, c.field2b());
   Expect.equals(499, c.field1c());
   Expect.equals(99, c.field1d());
   Expect.equals(499, _field1FromNewC4());
   Expect.equals(42, _field2FromNewC4());
 }