Store offsets in ModifierBuilders.

pkg/front_end/lib/src/fasta/builder/modifier_builder.dart

Index: pkg/front_end/lib/src/fasta/builder/modifier_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/modifier_builder.dart b/pkg/front_end/lib/src/fasta/builder/modifier_builder.dart
index 70f99c289571ace15a908a02f53600e99fc183ce..d03aa446e0e87101759b47f64e6ab15718213af8 100644
--- a/pkg/front_end/lib/src/fasta/builder/modifier_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/modifier_builder.dart
@@ -10,7 +10,9 @@ import '../modifier.dart'
 import 'builder.dart' show Builder;
 
 abstract class ModifierBuilder extends Builder {
-  ModifierBuilder(Builder parent, int charOffset, [Uri fileUri])
+  final int charOffset;
+
+  ModifierBuilder(Builder parent, this.charOffset, [Uri fileUri])
       : super(parent, charOffset, fileUri ?? parent?.fileUri);
 
   int get modifiers;