Fix crash in DillClassBuilder.isAbstract.

pkg/front_end/lib/src/fasta/dill/dill_class_builder.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.
 
 library fasta.dill_class_builder;
 
 import 'package:kernel/ast.dart' show
     Class,
     Constructor,
     Member,
     Procedure,
     ProcedureKind;
 
 import '../kernel/kernel_builder.dart' show
     Builder,
     KernelClassBuilder;
 
+import '../modifier.dart' show
+    abstractMask;
+
 import 'dill_member_builder.dart' show
     DillMemberBuilder;
 
 import 'dill_library_builder.dart' show
     DillLibraryBuilder;
 
 class DillClassBuilder extends KernelClassBuilder {
   final Class cls;
 
   final Map<String, Builder> constructors = <String, Builder>{};
 
   DillClassBuilder(Class cls, DillLibraryBuilder parent)
       : cls = cls,
-        super(null, null, cls.name, null, null, null, <String, Builder>{},
-            parent, cls.fileOffset);
+        super(null, computeModifiers(cls), cls.name, null, null, null,
+            <String, Builder>{}, parent, cls.fileOffset);
 
   void addMember(Member member) {
     DillMemberBuilder builder = new DillMemberBuilder(member, this);
     String name = member.name.name;
     if (member is Constructor ||
         (member is Procedure && member.kind == ProcedureKind.Factory)) {
       constructors[name] = builder;
     } else {
       DillMemberBuilder existing = members[name];
       if (existing == null) {
         members[name] = builder;
       } else {
         existing.next = builder;
       }
     }
   }
 
   Builder findConstructorOrFactory(String name) => constructors[name];
 }
+
+int computeModifiers(Class cls) {
+  return cls.isAbstract ? abstractMask : 0;
+}