Extract ResolutionWorldBuilderBase from ElementResolutionWorldBuilder

pkg/compiler/lib/src/common_elements.dart

 // Copyright (c) 2014, 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.
 
 // TODO(sigmund): rename and move to common/elements.dart
 library dart2js.type_system;
 
 import 'common/names.dart' show Uris;
 import 'elements/types.dart';
 import 'elements/entities.dart';
@@ skipping 234 matching lines @@
   /// Calls [f] for each class member declared or inherited in [cls] together
   /// with the class that declared the member.
   ///
   /// TODO(johnniwinther): This should not include static members of
   /// superclasses.
   void forEachClassMember(
       ClassEntity cls, void f(ClassEntity declarer, MemberEntity member));
 
   /// Returns the declared superclass of [cls].
   ///
-  /// Unnamed mixin applications are skipped, for instance for these classes
+  /// Unnamed mixin applications are included, for instance for these classes
   ///
   ///     class S {}
   ///     class M {}
   ///     class C extends S with M {}
   ///
-  /// the result of `getSuperClass(C)` is `S` and not the unnamed mixin
-  /// application typically named `S+M`.
+  /// the result of `getSuperClass(C)` is the unnamed mixin application
+  /// typically named `S+M` and `getSuperClass(S+M)` is `S`.
   ClassEntity getSuperClass(ClassEntity cls);
 
+  void forEachSupertype(ClassEntity cls, void f(InterfaceType supertype));
+
   /// Calls [f] for each class that is mixed into [cls] or one of its
   /// superclasses.
   void forEachMixin(ClassEntity cls, void f(ClassEntity mixin));
 
   /// Create the instantiation of [cls] with the given [typeArguments].
   InterfaceType createInterfaceType(
       ClassEntity cls, List<DartType> typeArguments);
 
   /// Returns the `dynamic` type.
   // TODO(johnniwinther): Remove this when `ResolutionDynamicType` is no longer
@@ skipping 291 matching lines @@
   }
 
   @override
   InterfaceType streamType([DartType elementType]) {
     if (elementType == null) {
       return getRawType(streamClass);
     }
     return createInterfaceType(streamClass, [elementType]);
   }
 }