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

pkg/compiler/lib/src/kernel/element_map.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.
 
 import 'package:kernel/ast.dart' as ir;
 
 import '../closure.dart';
 import '../common.dart';
 import '../constants/values.dart';
 import '../common_elements.dart';
@@ skipping 192 matching lines @@
 enum MemberKind {
   // A regular member defined by an [ir.Node].
   regular,
   // A constructor whose initializer is defined by an [ir.Constructor] node.
   constructor,
   // A constructor whose body is defined by an [ir.Constructor] node.
   constructorBody,
   // A closure class `call` method whose body is defined by an
   // [ir.FunctionExpression].
   closureCall,
+  // A field corresponding to a captured variable in the closure. It does not
+  // have a corresponding ir.Node.
+  closureField,
 }
 
 /// Definition information for a [MemberEntity].
 abstract class MemberDefinition {
   /// The defined member.
   MemberEntity get member;
 
   /// The kind of the defined member. This determines the semantics of [node].
   MemberKind get kind;
 
@@ skipping 198 matching lines @@
   /// [closureClassMaps].
   CapturedLoopScope getCapturedLoopScope(
       ClosureDataLookup closureLookup, ir.TreeNode node);
 }
 
 /// Comparator for the canonical order or named arguments.
 // TODO(johnniwinther): Remove this when named parameters are sorted in dill.
 int namedOrdering(ir.VariableDeclaration a, ir.VariableDeclaration b) {
   return a.name.compareTo(b.name);
 }