Prepare for separate setter scope.

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 'package:kernel/ast.dart' show Member;
 
 import '../dill/dill_member_builder.dart' show DillMemberBuilder;
 
 import '../errors.dart' show internalError;
 
 import '../messages.dart' show warning;
 
 import 'builder.dart' show Builder, LibraryBuilder, MemberBuilder;
 
 class PrefixBuilder extends Builder {
   final String name;
 
   final Map<String, Builder> exports;
 
   final LibraryBuilder parent;
 
   PrefixBuilder(this.name, this.exports, LibraryBuilder parent, int charOffset)
       : parent = parent,
         super(parent, charOffset, parent.fileUri);
 
+  Builder lookup(String name, int charOffset, Uri fileUri) {
+    return exports[name];
+  }
+
   Member findTopLevelMember(String name) {
     // TODO(ahe): Move this to KernelPrefixBuilder.
     Builder builder = exports[name];
     if (builder == null) {
       warning(
           parent.fileUri, -1, "'${this.name}' has no member named '$name'.");
     }
     if (builder is DillMemberBuilder) {
       return builder.member.isInstanceMember
           ? internalError("Unexpected instance member in export scope")
           : builder.member;
     } else if (builder is MemberBuilder) {
       return builder.target;
     } else {
       return null;
     }
   }
 
   @override
   String get fullNameForErrors => name;
 }