Handle private fields from mixins.

tests/language/private_mixin2_test.dart

+// Copyright (c) 2013, 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 on mixins.
+
+library private_mixin2;
+
+import 'private_other_mixin2.lib';
+
+void main() {
+  var c1 = new C1();
+  Expect.throws(() => c1._field, (e) => e is NoSuchMethodError);
+  Expect.throws(() => c1.field, (e) => e is NoSuchMethodError);
+
+  var c2 = new C2();
+  Expect.throws(() => c2._field, (e) => e is NoSuchMethodError);
+  Expect.equals(42, c2.field);
+}