Improve position information.

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

Index: pkg/front_end/lib/src/fasta/builder/prefix_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/prefix_builder.dart b/pkg/front_end/lib/src/fasta/builder/prefix_builder.dart
index de6b474dade28d15325626a539828d10e9a25bcc..38c54c82ef13e3c1ba493d4a126de508ff409520 100644
--- a/pkg/front_end/lib/src/fasta/builder/prefix_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/prefix_builder.dart
@@ -15,8 +15,12 @@ class PrefixBuilder extends Builder {
 
   final bool deferred;
 
+  @override
+  final int charOffset;
+
   PrefixBuilder(this.name, this.deferred, LibraryBuilder parent, int charOffset)
       : parent = parent,
+        charOffset = charOffset,
         super(parent, charOffset, parent.fileUri);
 
   Builder lookup(String name, int charOffset, Uri fileUri) {