anchor.diff:  /// Visitor that creates [jsAst.Expression]s for constants that are inlined
   /// and therefore can be created during modular code generation.
   class ModularConstantEmitter
  -    implements ConstantValueVisitor<jsAst.Expression, Null> {
  +    implements ConstantValueVisitor<jsAst.Expression?, Null> {
     final CompilerOptions _options;
     final ModularNamer _namer;
   
  
positive.code:       this._fieldAnalysis,
        this._emitter,
        this._constantReferenceGenerator,
        this._makeConstantList)
        : super(options, _namer);
  
    @override
    jsAst.Expression visitList(ListConstantValue constant, [_]) {
  
negative.code: // Copyright (c) 2012, 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.
  
  import '../common.dart';
  import '../common/elements.dart';
  