Support user generated custom native JS classes.

pkg/compiler/lib/src/common/backend_api.dart

Index: pkg/compiler/lib/src/common/backend_api.dart
diff --git a/pkg/compiler/lib/src/common/backend_api.dart b/pkg/compiler/lib/src/common/backend_api.dart
index b4425f5a33c4836764956478cdabb55aeaae3731..9f1d514daa37abc205e428046d61cc11a0da5d04 100644
--- a/pkg/compiler/lib/src/common/backend_api.dart
+++ b/pkg/compiler/lib/src/common/backend_api.dart
@@ -47,7 +47,7 @@ import '../library_loader.dart' show
 import '../native/native.dart' as native show
     NativeEnqueuer;
 import '../patch_parser.dart' show
-    checkNativeAnnotation;
+    checkNativeAnnotation, checkJsInteropAnnotation;
 import '../resolution/tree_elements.dart' show
     TreeElements;
 import '../tree/tree.dart' show
@@ -296,6 +296,16 @@ abstract class Backend {
         }
       });
     }
+    checkJsInteropAnnotation(compiler, library);
+    library.forEachLocalMember((Element element) {
+      checkJsInteropAnnotation(compiler, element);
+      if (element.isClass && element.isJsInterop) {
+        ClassElement classElement = element;
+        classElement.forEachMember((_, memberElement) {
+          checkJsInteropAnnotation(compiler, memberElement);
+        });
+      }
+    });
     return new Future.value();
   }