Prepare for separate setter scope.

pkg/front_end/lib/src/fasta/builder/prefix_builder.dart

Index: pkg/front_end/lib/src/fasta/builder/prefix_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/prefix_builder.dart b/pkg/front_end/lib/src/fasta/builder/prefix_builder.dart
index 27d93ee054e09bf189651d8ab4aab12c689437da..9ea1c43fec502bcffada9ea998339411c6932b21 100644
--- a/pkg/front_end/lib/src/fasta/builder/prefix_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/prefix_builder.dart
@@ -25,6 +25,10 @@ class PrefixBuilder extends Builder {
       : 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];