Extract OrderedTypeSetBuilderBase from OrderedTypeSetBuilder

pkg/compiler/lib/src/elements/modelx.dart

 // Copyright (c) 2013, 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 elements.modelx;
 
 import '../common.dart';
 import '../common/names.dart' show Identifiers;
 import '../common/resolution.dart' show Resolution, ParsingContext;
 import '../compiler.dart' show Compiler;
@@ skipping 2684 matching lines @@
     with
         AstElementMixin,
         AnalyzableElementX,
         ClassElementCommon,
         TypeDeclarationElementX<ResolutionInterfaceType>,
         PatchMixin<ClassElement>,
         ClassMemberMixin
     implements ClassElement {
   final int id;
 
-  ResolutionDartType supertype;
+  ResolutionInterfaceType supertype;
   Link<ResolutionDartType> interfaces;
   int supertypeLoadState;
   int resolutionState;
   bool isProxy = false;
   bool hasIncompleteHierarchy = false;
 
   OrderedTypeSet allSupertypesAndSelf;
 
   BaseClassElementX(String name, Element enclosing, this.id, int initialState)
       : supertypeLoadState = initialState,
@@ skipping 774 matching lines @@
       body = node.asFunctionExpression().body;
     }
     return new ParsedResolvedAst(
         declaration,
         node,
         body,
         definingElement.treeElements,
         definingElement.compilationUnit.script.resourceUri);
   }
 }