Prepare for separate setter scope.

pkg/front_end/lib/src/fasta/builder/library_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.library_builder;
 
 import '../combinator.dart' show Combinator;
 
 import '../errors.dart' show InputError, internalError, printUnexpected;
 
@@ skipping 90 matching lines @@
   ///
   /// If [constructorName] is null or the empty string, it's assumed to be an
   /// unnamed constructor.
   Builder getConstructor(String className,
       {String constructorName, bool isPrivate: false}) {
     constructorName ??= "";
     Builder cls = (isPrivate ? members : exports)[className];
     if (cls is ClassBuilder) {
       // TODO(ahe): This code is similar to code in `endNewExpression` in
       // `body_builder.dart`, try to share it.
-      Builder constructor = cls.findConstructorOrFactory(constructorName);
+      Builder constructor =
+          cls.findConstructorOrFactory(constructorName, -1, null);
       if (constructor == null) {
         // Fall-through to internal error below.
       } else if (constructor.isConstructor) {
         if (!cls.isAbstract) {
           return constructor;
         }
       } else if (constructor.isFactory) {
         return constructor;
       }
     }
     throw internalError("Internal error: No constructor named"
         " '$className::$constructorName' in '$uri'.");
   }
 
   int finishTypeVariables(ClassBuilder object) => 0;
 
   void becomeCoreLibrary(dynamicType, voidType) {
     addBuilder("dynamic",
         new DynamicTypeBuilder<T, dynamic>(dynamicType, this, -1), -1);
     addBuilder("void", new VoidTypeBuilder<T, dynamic>(voidType, this, -1), -1);
   }
+
+  void forEach(void f(String name, Builder builder)) {
+    members.forEach(f);
+  }
+
+  /// Don't use for scope lookup. Only use when an element is known to exist
+  /// (and not a setter).
+  Builder operator [](String name) {
+    return members[name] ?? internalError("Not found: '$name'.");
+  }
+
+  Builder lookup(String name, int charOffset, Uri fileUri) {
+    return members[name];
+  }
 }