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

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,
     MemberBuilder;
 
 import 'package:kernel/ast.dart' show
     Member;
 
 import '../dill/dill_member_builder.dart' show
     DillMemberBuilder;
 
 import '../errors.dart' show
     internalError;
 
 class PrefixBuilder extends Builder {
   final String name;
 
   final Map<String, Builder> exports;
 
   final LibraryBuilder parent;
 
-  PrefixBuilder(this.name, this.exports, this.parent);
+  PrefixBuilder(this.name, this.exports, LibraryBuilder parent, int charOffset)
+      : parent = parent, super(parent, charOffset, parent.fileUri);
 
   Member findTopLevelMember(String name) {
     // TODO(ahe): Move this to KernelPrefixBuilder.
     Builder builder = exports[name];
     if (builder == null) {
       // TODO(ahe): Report error?
       print("${this.name} has no member named $name");
     }
     if (builder is DillMemberBuilder) {
       return builder.member.isInstanceMember
@@ skipping 15 matching lines @@
         if (existing != null) {
           member = existing.combineAmbiguousImport(name, member, library);
         }
         exports[name] = member;
       });
       return this;
     }
     return super.combineAmbiguousImport(name, other, library);
   }
 }