Revert "Store actual Reference(s) for additional exports."

pkg/front_end/lib/src/fasta/scope.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.scope;
 
 import 'builder/builder.dart' show Builder, TypeVariableBuilder;
 
 import 'fasta_codes.dart'
     show
@@ skipping 262 matching lines @@
   ScopeBuilder(this.scope);
 
   void addMember(String name, Builder builder) {
     scope.local[name] = builder;
   }
 
   void addSetter(String name, Builder builder) {
     scope.setters[name] = builder;
   }
 
-  Builder operator [](String name) {
-    return scope.local[name] ?? scope.setters[name];
-  }
+  Builder operator [](String name) => scope.local[name];
 }
 
 abstract class ProblemBuilder extends Builder {
   final String name;
 
   final Builder builder;
 
   ProblemBuilder(this.name, this.builder, int charOffset, Uri fileUri)
       : super(null, charOffset, fileUri);
 
@@ skipping 37 matching lines @@
 
   Message get message => templateAccessError.withArguments(name);
 }
 
 class AmbiguousBuilder extends ProblemBuilder {
   AmbiguousBuilder(String name, Builder builder, int charOffset, Uri fileUri)
       : super(name, builder, charOffset, fileUri);
 
   Message get message => templateDuplicatedName.withArguments(name);
 }