Improve compile-time error handling.

pkg/front_end/lib/src/fasta/import.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.import;
 
 import 'builder/builder.dart' show
     Builder,
     LibraryBuilder,
     PrefixBuilder;
@@ skipping 38 matching lines @@
     imported.exports.forEach((String name, Builder member) {
       if (combinators != null) {
         for (Combinator combinator in combinators) {
           if (combinator.isShow && !combinator.names.contains(name)) return;
           if (combinator.isHide && combinator.names.contains(name)) return;
         }
       }
       add(name, member);
     });
     if (prefix != null) {
-      Builder existing = importer.addBuilder(prefix.name, prefix);
+      Builder existing = importer.addBuilder(prefix.name, prefix, charOffset);
       if (existing == prefix) {
         importer.addToScope(prefix.name, prefix);
       }
     }
   }
 }