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

pkg/front_end/lib/src/fasta/builder/member_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.member_builder;
 
 import 'builder.dart' show
     Builder,
     ClassBuilder,
     ModifierBuilder;
 
 abstract class MemberBuilder extends ModifierBuilder {
+  /// For top-level members, the parent is set correctly during
+  /// construction. However, for class members, the parent is initially the
+  /// library and updated later.
   Builder parent;
 
+  MemberBuilder(Builder parent, int charOffset)
+      : parent = parent, super(parent, charOffset);
+
   bool get isInstanceMember => isClassMember && !isStatic;
 
   bool get isClassMember => parent is ClassBuilder;
 
   bool get isTopLevel => !isClassMember;
 }