Improve position information.

pkg/front_end/lib/src/fasta/source/source_library_builder.dart

Index: pkg/front_end/lib/src/fasta/source/source_library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/source/source_library_builder.dart b/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
index 31b508b011350579af4c38c9214e85e9f11c7b5e..30932735a13c6fb4d83a7ec703202001163f2b13 100644
--- a/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_library_builder.dart
@@ -149,7 +149,7 @@ abstract class SourceLibraryBuilder<T extends TypeBuilder, R>
         prefixCharOffset));
   }
 
-  void addPart(List<MetadataBuilder> metadata, String path) {
+  void addPart(List<MetadataBuilder> metadata, String path, int charOffset) {
     Uri resolvedUri;
     Uri newFileUri;
     if (uri.scheme == "dart") {