Make sure the mixin class is finalized immediately (and not some time later)

tests/language/regress_22719_test.dart

+// Copyright (c) 2015, 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:collection';
+import 'package:expect/expect.dart';
+
+abstract class A {
+}
+
+abstract class B extends Object with IterableMixin<int> {
+  Iterator<int> get iterator;
+}
+
+abstract class C extends A with IterableMixin<int> implements B {
+  final list = [1,2,3,4,5];
+  Iterator<int> get iterator => list.iterator;
+}
+
+class D extends C {
+}
+
+void main() {
+  var d = new D();
+  var expected = 1;
+  for(var i in d) {
+    Expect.equals(expected, i);
+    expected += 1;
+  }
+}
+