anchor.diff:  }
   
   class _NullLiteral extends Expression {
  -  const _NullLiteral();
  +  _NullLiteral();
   
     @override
     String toString() => 'null';
  
positive.code: import 'package:_fe_analyzer_shared/src/flow_analysis/flow_analysis.dart';
  import 'package:test/test.dart';
  
  const Expression nullLiteral = const _NullLiteral();
  
  Statement assert_(Expression condition, [Expression? message]) =>
      new _Assert(condition, message);
  
negative.code: /// Representation of an expression or statement in the pseudo-Dart language
  /// used for flow analysis testing.
  class Node {
    static int _nextId = 0;
  
    final int id;
  
    Node._() : id = _nextId++;
  
    String toString() => 'Node#$id';
  }
  
  /// Helper class allowing tests to examine the values of variables' SSA nodes.
  