Improve position information.

pkg/front_end/lib/src/fasta/builder/prefix_builder.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.prefix_builder;
 
 import 'builder.dart' show Builder, LibraryBuilder, Scope;
 
 class PrefixBuilder extends Builder {
   final String name;
 
   final Scope exports = new Scope.top();
 
   final LibraryBuilder parent;
 
   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) {
     return exports.lookup(name, charOffset, fileUri);
   }
 
   @override
   String get fullNameForErrors => name;
 }