Allow native classes to mixin behavior from an ordinary Dart class.

sdk/lib/_internal/compiler/implementation/native_handler.dart

Index: sdk/lib/_internal/compiler/implementation/native_handler.dart
diff --git a/sdk/lib/_internal/compiler/implementation/native_handler.dart b/sdk/lib/_internal/compiler/implementation/native_handler.dart
index f0c82013a66f3118a424fe816480dfbae1461e03..e3e7bbe1319bfab8cd6233c16c7b014333cec8f1 100644
--- a/sdk/lib/_internal/compiler/implementation/native_handler.dart
+++ b/sdk/lib/_internal/compiler/implementation/native_handler.dart
@@ -437,8 +437,14 @@ class NativeCodegenEnqueuer extends NativeEnqueuerBase {
       subtypes.add(cls);
     }
 
+    ClassElement superclass = cls.superclass;
+    while (superclass != null && superclass.isMixinApplication) {
+      assert(!superclass.isNative());
+      superclass = superclass.superclass;
+    }
+
     List<Element> directSubtypes = emitter.directSubtypes.putIfAbsent(
-        cls.superclass,
+        superclass,
         () => <ClassElement>[]);
     directSubtypes.add(cls);
   }