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

pkg/compiler/lib/src/ssa/builder_kernel.dart

 // Copyright (c) 2016, 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 '../common/codegen.dart' show CodegenRegistry;
 import '../common/names.dart';
@@ skipping 152 matching lines @@
         case MemberKind.constructor:
           ir.Constructor constructor = definition.node;
           _targetFunction = constructor.function;
           buildConstructor(constructor);
           break;
         case MemberKind.constructorBody:
           ir.Constructor constructor = definition.node;
           _targetFunction = constructor.function;
           buildConstructorBody(constructor);
           break;
+        case MemberKind.closureField:
+          failedAt(targetElement, "Unexpected closure field: $targetElement");
+          break;
       }
       assert(graph.isValid());
       if (_targetFunction != null) {
         _ensureDefaultArgumentValues(_targetFunction);
       }
       return graph;
     });
   }
 
   void _ensureDefaultArgumentValues(ir.FunctionNode function) {
@@ skipping 3470 matching lines @@
     enterBlock.setBlockFlow(
         new HTryBlockInformation(
             kernelBuilder.wrapStatementGraph(bodyGraph),
             exception,
             kernelBuilder.wrapStatementGraph(catchGraph),
             kernelBuilder.wrapStatementGraph(finallyGraph)),
         exitBlock);
     kernelBuilder.inTryStatement = previouslyInTryStatement;
   }
 }