Ensure locations are always provided, but don't store them yet.

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 7dd8b2b2bb9b145931e6d8636951c4fd0bbaf999..c60c0b33454db365422087f01c781f97bd0177e8 100644
--- a/pkg/front_end/lib/src/fasta/builder/modifier_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/modifier_builder.dart
@@ -15,6 +15,9 @@ import 'builder.dart' show
     Builder;
 
 abstract class ModifierBuilder extends Builder {
+  ModifierBuilder(Builder parent, int charOffset, [Uri fileUri])
+      : super(parent, charOffset, fileUri ?? parent?.fileUri);
+
   int get modifiers;
 
   bool get isAbstract => (modifiers & abstractMask) != 0;