anchor.diff:  /// analysis testing.  Methods in this class may be used to create more complex
   /// expressions based on this one.
   abstract class Expression extends Node implements _Visitable<Type> {
  -  const Expression() : super._();
  +  Expression() : super._();
   
     /// If `this` is an expression `x`, creates the expression `x!`.
     Expression get nonNullAssert => new _NonNullAssert(this);
  
positive.code: /// Representation of an expression or statement in the pseudo-Dart language
  /// used for flow analysis testing.
  class Node {
    const Node._();
  }
  
  /// Helper class allowing tests to examine the values of variables' SSA nodes.
  
negative.code: }
  
  class _NullLiteral extends Expression {
    _NullLiteral();
  
    @override
    String toString() => 'null';
  