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

pkg/dev_compiler/test/codegen/language/mixin_abstract_getter_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 'package:expect/expect.dart';
+
+abstract class B {
+  int get x;
+}
+
+class C {
+  int get x => 42;
+}
+
+class D extends C with B {
+  final int x;
+
+  D(this.x);
+}
+
+
+class C2 {
+  int get x => 42;
+}
+
+abstract class B2 extends C2 {
+  int get x;
+}
+
+class D2 extends B2 {
+  final int x;
+
+  D2(this.x);
+}
+
+
+void main() {
+  var d = new D(17);
+  Expect.equals(d.x, 17);
+
+  var d2 = new D2(17);
+  Expect.equals(d.x, 17);
+}