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

pkg/dev_compiler/test/codegen/lib/math/implement_rectangle_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.
+
 import 'dart:math' hide Rectangle;
 import 'dart:math' as math show Point, Rectangle, MutableRectangle;
 import 'package:expect/expect.dart' show Expect;
 
 void main() {
   verifyRectable(new Rectangle(1, 2, 3, 4));
 }
 
 void verifyRectable(math.Rectangle rect) {
   Expect.equals(1.0, rect.left.toDouble());
@@ skipping 56 matching lines @@
   }
 
   Rectangle<T> intersection(math.Rectangle<T> rect) {
     T rLeft = max(left, rect.left);
     T rTop = max(top, rect.top);
     T rRight = min(right, rect.right);
     T rBottom = min(bottom, rect.bottom);
     return new Rectangle<T>(rLeft, rTop, rRight - rLeft, rBottom - rTop);
   }
 }