Support user generated custom native JS classes.

pkg/compiler/lib/src/native/enqueue.dart

Index: pkg/compiler/lib/src/native/enqueue.dart
diff --git a/pkg/compiler/lib/src/native/enqueue.dart b/pkg/compiler/lib/src/native/enqueue.dart
index 1f9b47b5d070e8b23178b0a348399a3fd4ea1bf6..18ef9d0bacdfa2e1edea3d04342964d8b816d11e 100644
--- a/pkg/compiler/lib/src/native/enqueue.dart
+++ b/pkg/compiler/lib/src/native/enqueue.dart
@@ -139,6 +139,7 @@ abstract class NativeEnqueuerBase implements NativeEnqueuer {
     if (backend.isolateHelperLibrary != null) {
       processNativeClassesInLibrary(backend.isolateHelperLibrary);
     }
+    libraries.forEach(processJsInteropAnnotationsInLibrary);
     processSubclassesOfNativeClasses(libraries);
     if (!enableLiveTypeAnalysis) {
       nativeClasses.forEach((c) => enqueueClass(c, 'forced'));
@@ -146,6 +147,13 @@ abstract class NativeEnqueuerBase implements NativeEnqueuer {
     }
   }
 
+  void processJsInteropAnnotationsInLibrary(LibraryElement library) {
+    checkJsInteropAnnotation(compiler, library);
+    library.implementation.forEachLocalMember((Element element) {
+      checkJsInteropAnnotation(compiler, element);
+    });
+  }
+
   void processNativeClassesInLibrary(LibraryElement library) {
     // Use implementation to ensure the inclusion of injected members.
     library.implementation.forEachLocalMember((Element element) {
@@ -608,6 +616,8 @@ class NativeResolutionEnqueuer extends NativeEnqueuerBase {
   void processNativeClass(ClassElement classElement) {
     super.processNativeClass(classElement);
 
+    // Js Interop interfaces do not have tags.
+    if (classElement.isJsInterop) return;
     // Since we map from dispatch tags to classes, a dispatch tag must be used
     // on only one native class.
     for (String tag in nativeTagsOfClass(classElement)) {