Store offsets in ModifierBuilders.

pkg/front_end/lib/src/fasta/builder/modifier_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.modifier_builder;
 
 import '../modifier.dart'
     show abstractMask, constMask, externalMask, finalMask, staticMask;
 
 import 'builder.dart' show Builder;
 
 abstract class ModifierBuilder extends Builder {
-  ModifierBuilder(Builder parent, int charOffset, [Uri fileUri])
+  final int charOffset;
+
+  ModifierBuilder(Builder parent, this.charOffset, [Uri fileUri])
       : super(parent, charOffset, fileUri ?? parent?.fileUri);
 
   int get modifiers;
 
   bool get isAbstract => (modifiers & abstractMask) != 0;
 
   bool get isConst => (modifiers & constMask) != 0;
 
   bool get isExternal => (modifiers & externalMask) != 0;
 
   bool get isFinal => (modifiers & finalMask) != 0;
 
   bool get isStatic => (modifiers & staticMask) != 0;
 }