Support mixins when loading from .dill

tests/compiler/dart2js/kernel/closed_world_from_dill_test.dart

Index: tests/compiler/dart2js/kernel/closed_world_from_dill_test.dart
diff --git a/tests/compiler/dart2js/kernel/closed_world_from_dill_test.dart b/tests/compiler/dart2js/kernel/closed_world_from_dill_test.dart
index 1c66cd642eaa81e45c92bd43b00444169cd447ae..6730ba8cb5a855ac9332a393e3f1d201368f2b96 100644
--- a/tests/compiler/dart2js/kernel/closed_world_from_dill_test.dart
+++ b/tests/compiler/dart2js/kernel/closed_world_from_dill_test.dart
@@ -31,8 +31,28 @@ 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();
 }
 '''
 };