Enqueue superclasses instead of supertypes.

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.backend_api;
 
 import 'dart:async' show Future;
 
 import '../compiler.dart' show
     Compiler;
@@ skipping 135 matching lines @@
                                 Element annotatedElement,
                                 Registry registry) {}
 
   /// Called to notify to the backend that a class is being instantiated.
   // TODO(johnniwinther): Remove this. It's only called once for each [cls] and
   // only with [Compiler.globalDependencies] as [registry].
   void registerInstantiatedClass(ClassElement cls,
                                  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) {}
 
   /// Register an is check to the backend.
   void registerIsCheckForCodegen(DartType type,
                                  Enqueuer enqueuer,
                                  Registry registry) {}
 
   /// Register a runtime type variable bound tests between [typeArgument] and
@@ skipping 195 matching lines @@
 
   void forgetElement(Element element) {}
 
   void registerMainHasArguments(Enqueuer enqueuer) {}
 
   void registerAsyncMarker(FunctionElement element,
                            Enqueuer enqueuer,
                            Registry registry) {}
 }