Support mixins when loading from .dill

tests/compiler/dart2js/kernel/closed_world_from_dill_test.dart

 // Copyright (c) 2017, 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.
 
 // Partial test that the closed world computed from [WorldImpact]s derived from
 // kernel is equivalent to the original computed from resolution.
 library dart2js.kernel.closed_world_from_dill_test;
 
 import 'dart:async';
 import 'dart:io';
@@ skipping 13 matching lines @@
 import 'package:expect/expect.dart';
 import '../memory_compiler.dart';
 import '../equivalence/check_functions.dart';
 import '../serialization/helper.dart';
 import 'test_helpers.dart';
 
 import 'compiler_helper.dart';
 
 const SOURCE = const {
   'main.dart': '''
+
+class ClassWithSetter {
+  void set setter(_) {}
+}
+
+class Mixin {
+  method1() {}
+  method2() {}
+  method3() {}
+}
+class Class1 = Object with Mixin;
+class Class2 extends Object with Mixin {
+  method3() {}
+}
+
 main() {
   print('Hello World');
+  ''.contains; // Trigger member closurization.
+  new ClassWithSetter().setter = null;
+  new Class1().method1();
+  new Class2().method2();
+  new Class2().method3();
 }
 '''
 };
 
 main(List<String> args) {
   asyncTest(() async {
     await mainInternal(args);
   });
 }
 
@@ skipping 67 matching lines @@
       enqueuer1, enqueuer2, elementEquivalence: equivalence.entityEquivalence,
       typeEquivalence: (ResolutionDartType a, DartType b) {
     return equivalence.typeEquivalence(unalias(a), b);
   }, elementFilter: elementFilter, verbose: arguments.verbose);
 
   checkClosedWorlds(closedWorld1, closedWorld2,
       strategy: equivalence.defaultStrategy, verbose: arguments.verbose);
 
   return ResultKind.success;
 }