Make the ClosedWorld build the closure class on the kernel side.

pkg/compiler/lib/src/js_model/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.
 
 library dart2js.js_model.elements;
 
 import '../elements/entities.dart';
 import '../elements/names.dart';
 import '../elements/types.dart';
 import '../kernel/elements.dart';
@@ skipping 296 matching lines @@
   JClass(this.library, this.classIndex, this.name, {this.isAbstract});
 
   @override
   bool get isClosure => false;
 
   String toString() => '${jsElementPrefix}class($name)';
 }
 
 abstract class JMember implements MemberEntity, IndexedMember {
   /// Member index used for fast lookup in [JsToFrontendMapImpl].
-  int _memberIndex;
+  final int memberIndex;
   final JLibrary library;
   final JClass enclosingClass;
   final Name _name;
   final bool _isStatic;
 
-  JMember(this._memberIndex, this.library, this.enclosingClass, this._name,
+  JMember(this.memberIndex, this.library, this.enclosingClass, this._name,
       {bool isStatic: false})
       : _isStatic = isStatic;
 
-  int get memberIndex => _memberIndex;
-
-  /// Should only be called by closure methods. All others should set
-  /// memberIndex at initialization time.
-  void set setClosureMemberIndex(int newIndex) {
-    _memberIndex = newIndex;
-  }
-
   String get name => _name.text;
 
   Name get memberName => _name;
 
   @override
   bool get isAssignable => false;
 
   @override
   bool get isConst => false;
 
@@ skipping 186 matching lines @@
   final Entity typeDeclaration;
   final String name;
   final int index;
 
   JTypeVariable(
       this.typeVariableIndex, this.typeDeclaration, this.name, this.index);
 
   String toString() =>
       '${jsElementPrefix}type_variable(${typeDeclaration.name}.$name)';
 }