Native subclasses of a mixin include classes extending native classes

sdk/lib/_internal/compiler/implementation/js_backend/backend.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
index e9ae5cc08a6235a3acd5f440a6680d078db21214..ee462946f04846d2f4c0d4a10d1139b14d9e73b4 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
@@ -452,7 +452,7 @@ class JavaScriptBackend extends Backend {
       Iterable<ClassElement> subclasses = compiler.world.subclassesOf(use);
       if (subclasses != null) {
         for (ClassElement subclass in subclasses) {
-          if (subclass.isNative()) {
+          if (Elements.isNativeOrExtendsNative(subclass)) {
             if (result == null) result = new Set<ClassElement>();
             result.add(subclass);
           }