Enqueue superclasses instead of supertypes.

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 71c86165d11cd1d5fea54d9666e27735e3153c70..b095b230809e7c65cb78c1e79b78df1a36adc1e8 100644
--- a/pkg/compiler/lib/src/common/backend_api.dart
+++ b/pkg/compiler/lib/src/common/backend_api.dart
@@ -153,6 +153,12 @@ abstract class Backend {
                                  Enqueuer enqueuer,
                                  Registry registry) {}
 
+  /// Called to notify to the backend that a class is implemented by an
+  /// instantiated class.
+  void registerImplementedClass(ClassElement cls,
+                                Enqueuer enqueuer,
+                                Registry registry) {}
+
   /// Called to notify to the backend that an interface type has been
   /// instantiated.
   void registerInstantiatedType(InterfaceType type, Registry registry) {}