Enqueue superclasses instead of supertypes.

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

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index 4273279bc712c7409c1dbd7845217dc98d7adff7..b5dffc084e242900a34d4fad6bcb77a17f3bfbeb 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -1027,6 +1027,18 @@ class JavaScriptBackend extends Backend {
   void registerInstantiatedClass(ClassElement cls,
                                  Enqueuer enqueuer,
                                  Registry registry) {
+    _processClass(cls, enqueuer, registry);
+  }
+
+  void registerImplementedClass(ClassElement cls,
+                                Enqueuer enqueuer,
+                                Registry registry) {
+    _processClass(cls, enqueuer, registry);
+  }
+
+  void _processClass(ClassElement cls,
+                     Enqueuer enqueuer,
+                     Registry registry) {
     if (!cls.typeVariables.isEmpty) {
       typeVariableHandler.registerClassWithTypeVariables(cls, enqueuer,
                                                          registry);