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 63b112e22c1c262a8afa981c1d3750eed2f80a6a..c73df58d2c5dfcde4ad82cf01cd9509152ac54ff 100644
--- a/pkg/compiler/lib/src/common/backend_api.dart
+++ b/pkg/compiler/lib/src/common/backend_api.dart
@@ -45,7 +45,7 @@ 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;
 
@@ -281,6 +281,11 @@ abstract class Backend {
         }
       });
     }
+    library.forEachLocalMember((Element element) {
+      if (element.isClass) {
+        checkJsInteropAnnotation(compiler, element);
+      }
+    });
     return new Future.value();
   }