fix #27643, failed to virtualize fields if an abstract one is mixed in

pkg/dev_compiler/test/codegen/lib/math/implement_rectangle_test.dart

Index: pkg/dev_compiler/test/codegen/lib/math/implement_rectangle_test.dart
diff --git a/pkg/dev_compiler/test/codegen/lib/math/implement_rectangle_test.dart b/pkg/dev_compiler/test/codegen/lib/math/implement_rectangle_test.dart
index ce93360e77f769224379130db27f72c69f3a7507..3a31ed8433134597b90ffe011fcfd8a3511de9e0 100644
--- a/pkg/dev_compiler/test/codegen/lib/math/implement_rectangle_test.dart
+++ b/pkg/dev_compiler/test/codegen/lib/math/implement_rectangle_test.dart
@@ -1,3 +1,7 @@
+// 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 'dart:math' hide Rectangle;
 import 'dart:math' as math show Point, Rectangle, MutableRectangle;
 import 'package:expect/expect.dart' show Expect;