Improve compile-time error handling.

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 16e6b43ba27ea5dc85ebf60ce8399734426fa94f..2fb52ae277ad3faaaa1211e1be93830799927246 100644
--- a/pkg/front_end/lib/src/fasta/builder/library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/library_builder.dart
@@ -47,15 +47,16 @@ abstract class LibraryBuilder<T extends TypeBuilder, R> extends Builder {
   LibraryBuilder(Uri fileUri)
       : super(null, -1, fileUri);
 
-  Builder addBuilder(String name, Builder builder);
+  Builder addBuilder(String name, Builder builder, int charOffset);
 
   void addExporter(LibraryBuilder exporter, List<Combinator> combinators,
       int charOffset) {
     exporters.add(new Export(exporter, this, combinators, charOffset));
   }
 
-  void addCompileTimeError(int charOffset, Object message) {
-    InputError error = new InputError(uri, charOffset, message);
+  void addCompileTimeError(int charOffset, Object message, [Uri fileUri]) {
+    fileUri ??= this.fileUri;
+    InputError error = new InputError(fileUri, charOffset, message);
     compileTimeErrors.add(error);
     print(error.format());
   }