Fix x.runtimeType for 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 de43ed6a51b660f4c0164575d27f069e13e4a1d5..7b81580ca65e5f0e726cf772c4df848970776dbc 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
@@ -979,6 +979,7 @@ class JavaScriptBackend extends Backend {
           Set<Element> set = interceptedElements.putIfAbsent(
               member.name, () => new Set<Element>());
           set.add(member);
+          if (classElement == jsInterceptorClass) return;
           if (!classElement.isNative()) {
             MixinApplicationElement mixinApplication = classElement;
             assert(member.getEnclosingClass() == mixinApplication.mixin);
@@ -1368,6 +1369,11 @@ class JavaScriptBackend extends Backend {
     return new native.NativeCodegenEnqueuer(world, compiler, emitter);
   }
 
+  ClassElement defaultSuperclass(ClassElement element) {
+    // Native classes inherit from Interceptor.
+    return element.isNative() ? jsInterceptorClass : compiler.objectClass;
+  }
+
   /**
    * Unit test hook that returns code of an element as a String.
    *