Extract ResolutionWorldBuilderBase from ElementResolutionWorldBuilder

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

 // Copyright (c) 2017, 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.
 
 /// Entity model for elements derived from Kernel IR.
 
 import '../elements/elements.dart';
 import '../elements/entities.dart';
 
 class KLibrary implements LibraryEntity {
   /// Library index used for fast lookup in [KernelWorldBuilder].
   final int libraryIndex;
   final String name;
   final Uri canonicalUri;
 
   KLibrary(this.libraryIndex, this.name, this.canonicalUri);
 
   String toString() => 'library($name)';
 }
 
 class KClass implements ClassEntity {
   final KLibrary library;
 
   /// Class index used for fast lookup in [KernelWorldBuilder].
   final int classIndex;
+
   final String name;
+  final bool isAbstract;
 
-  KClass(this.library, this.classIndex, this.name);
+  KClass(this.library, this.classIndex, this.name, {this.isAbstract});
 
   @override
   bool get isClosure => false;
 
   String toString() => 'class($name)';
 }
 
 abstract class KMember implements MemberEntity {
   /// Member index used for fast lookup in [KernelWorldBuilder].
   final int memberIndex;
@@ skipping 167 matching lines @@
 class KLocalFunction implements Local {
   final String name;
   final MemberEntity memberContext;
   final Entity executableContext;
 
   KLocalFunction(this.name, this.memberContext, this.executableContext);
 
   String toString() =>
       'local_function(${memberContext.name}.${name ?? '<anonymous>'})';
 }