Don't try to add abstract methods to the codegen work queue to generate bodies

pkg/compiler/lib/src/kernel/kernel_backend_strategy.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.kernel.backend_strategy;
 
 import 'package:kernel/ast.dart' as ir;
 
 import '../backend_strategy.dart';
 import '../common.dart';
@@ skipping 33 matching lines @@
 class KernelCodegenWorkItemBuilder implements WorkItemBuilder {
   final JavaScriptBackend _backend;
   final ClosedWorld _closedWorld;
 
   KernelCodegenWorkItemBuilder(this._backend, this._closedWorld);
 
   CompilerOptions get _options => _backend.compiler.options;
 
   @override
   CodegenWorkItem createWorkItem(MemberEntity entity) {
+    if (entity.isAbstract) return null;
+
     // Codegen inlines field initializers. It only needs to generate
     // code for checked setters.
     if (entity.isField && entity.isInstanceMember) {
       if (!_options.enableTypeAssertions || entity.enclosingClass.isClosure) {
         return null;
       }
     }
 
     return new KernelCodegenWorkItem(_backend, _closedWorld, entity);
   }
@@ skipping 202 matching lines @@
     return sorted;
   }
 
   @override
   Iterable<TypedefEntity> sortTypedefs(Iterable<TypedefEntity> typedefs) {
     // TODO(redemption): Support this.
     assert(typedefs.isEmpty);
     return typedefs;
   }
 }