Extract OrderedTypeSetBuilderBase from OrderedTypeSetBuilder

pkg/compiler/lib/src/serialization/modelz.dart

 // Copyright (c) 2015, 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.
 
 /// Implementation of the element model used for deserialiation.
 ///
 /// These classes are created by [ElementDeserializer] triggered by the
 /// [Deserializer].
 
 library dart2js.serialization.modelz;
@@ skipping 914 matching lines @@
         AnalyzableElementMixin,
         AstElementMixinZ,
         ClassElementCommon,
         class_members.ClassMemberMixin,
         ContainerMixin,
         LibraryMemberMixin,
         TypeDeclarationMixin<ResolutionInterfaceType>,
         ClassElementMixin
     implements ClassElement {
   bool _isObject;
-  ResolutionDartType _supertype;
+  ResolutionInterfaceType _supertype;
   OrderedTypeSet _allSupertypesAndSelf;
   Link<ResolutionDartType> _interfaces;
   ResolutionFunctionType _callType;
 
   ClassElementZ(ObjectDecoder decoder) : super(decoder);
 
   @override
   List<ResolutionDartType> _getTypeVariables() {
     return _decoder.getTypes(Key.TYPE_VARIABLES, isOptional: true);
   }
@@ skipping 1465 matching lines @@
   }
 
   @override
   Node get node => throw new UnsupportedError('${this}.node');
 
   @override
   bool get hasNode => false;
 
   String toString() => 'MetadataAnnotationZ(${constant.toDartText()})';
 }