anchor.diff:  
   
   class AstFactory {
  -  EmptyExpression empty() => new EmptyExpression();
  +  EmptyExpression empty() => const EmptyExpression();
   
     Literal literal(v) => new Literal(v);
   
  
positive.code: 
  // Helper functions for building expression trees programmatically
  
  EmptyExpression empty() => new EmptyExpression();
  Literal literal(v) => new Literal(v);
  MapLiteral mapLiteral(List<MapLiteralEntry> entries) => new MapLiteral(entries);
  MapLiteralEntry mapLiteralEntry(Literal key, Expression value) =>
  
negative.code: 
  /// Base class for all expressions
  abstract class Expression {
    const Expression();
    accept(Visitor v);
  }
  
  class EmptyExpression extends Expression {
    const EmptyExpression();
    accept(Visitor v) => v.visitEmptyExpression(this);
  }
  
  class Literal<T> extends Expression {
  