Fix crash in DillClassBuilder.isAbstract.

pkg/front_end/lib/src/fasta/dill/dill_class_builder.dart

Index: pkg/front_end/lib/src/fasta/dill/dill_class_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/dill/dill_class_builder.dart b/pkg/front_end/lib/src/fasta/dill/dill_class_builder.dart
index 0211cc4b197c6f6d549e48896a4486ddae0b5cf5..4c97dd2006e9aaa8b22ce1ef1f4a852a44e4b919 100644
--- a/pkg/front_end/lib/src/fasta/dill/dill_class_builder.dart
+++ b/pkg/front_end/lib/src/fasta/dill/dill_class_builder.dart
@@ -15,6 +15,9 @@ import '../kernel/kernel_builder.dart' show
     Builder,
     KernelClassBuilder;
 
+import '../modifier.dart' show
+    abstractMask;
+
 import 'dill_member_builder.dart' show
     DillMemberBuilder;
 
@@ -28,8 +31,8 @@ class DillClassBuilder extends KernelClassBuilder {
 
   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);
@@ -49,3 +52,7 @@ class DillClassBuilder extends KernelClassBuilder {
 
   Builder findConstructorOrFactory(String name) => constructors[name];
 }
+
+int computeModifiers(Class cls) {
+  return cls.isAbstract ? abstractMask : 0;
+}