Revert "Revert "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 6dcad8db1106a21deb79de2dfaece4baf2e7f6e4..7feb253ee3c6375dd5dbdd799c38e770855572b9 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
@@ -959,6 +959,7 @@ class JavaScriptBackend extends Backend {
   void validateInterceptorImplementsAllObjectMethods(
       ClassElement interceptorClass) {
     if (interceptorClass == null) return;
+    interceptorClass.ensureResolved(compiler);
     compiler.objectClass.forEachMember((_, Element member) {
       if (member.isGenerativeConstructor()) return;
       Element interceptorMember = interceptorClass.lookupMember(member.name);
@@ -978,6 +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);
@@ -1367,6 +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.
    *