Add @anonymous annotation and restrict object literal constructors to only anonymous classes. This …

pkg/compiler/lib/src/js_backend/backend_helpers.dart

Index: pkg/compiler/lib/src/js_backend/backend_helpers.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend_helpers.dart b/pkg/compiler/lib/src/js_backend/backend_helpers.dart
index 899d65eb8d261a86746b7d023c0e95f26d74f629..2650a7e940ee73f8125ba8af90f832b34ed66c79 100644
--- a/pkg/compiler/lib/src/js_backend/backend_helpers.dart
+++ b/pkg/compiler/lib/src/js_backend/backend_helpers.dart
@@ -130,6 +130,7 @@ class BackendHelpers {
   ClassElement irRepresentationClass;
 
   ClassElement jsAnnotationClass;
+  ClassElement jsAnonymousClass;
 
   Element getInterceptorMethod;
 
@@ -303,6 +304,7 @@ class BackendHelpers {
       typedArrayOfIntClass = findClass('NativeTypedArrayOfInt');
     } else if (uri == PACKAGE_JS) {
       jsAnnotationClass = find(library, 'JS');
+      jsAnonymousClass = find(library, '_Anonymous');
     }
   }