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

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

Index: pkg/front_end/lib/src/fasta/builder/library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/library_builder.dart b/pkg/front_end/lib/src/fasta/builder/library_builder.dart
index 1d3476eb48614baa2fb118661b90bdc3452b37f4..16e6b43ba27ea5dc85ebf60ce8399734426fa94f 100644
--- a/pkg/front_end/lib/src/fasta/builder/library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/library_builder.dart
@@ -44,10 +44,14 @@ abstract class LibraryBuilder<T extends TypeBuilder, R> extends Builder {
 
   Map<String, Builder> get exports;
 
+  LibraryBuilder(Uri fileUri)
+      : super(null, -1, fileUri);
+
   Builder addBuilder(String name, Builder builder);
 
-  void addExporter(LibraryBuilder exporter, List<Combinator> combinators) {
-    exporters.add(new Export(exporter, this, combinators));
+  void addExporter(LibraryBuilder exporter, List<Combinator> combinators,
+      int charOffset) {
+    exporters.add(new Export(exporter, this, combinators, charOffset));
   }
 
   void addCompileTimeError(int charOffset, Object message) {
@@ -61,7 +65,7 @@ abstract class LibraryBuilder<T extends TypeBuilder, R> extends Builder {
   void addToScope(String name, Builder member);
 
   InvalidTypeBuilder buildAmbiguousBuilder(
-      String name, Builder builder, Builder other);
+      String name, Builder builder, Builder other, int charOffset);
 
   int finishStaticInvocations() => 0;
 }