anchor.diff:  
     bool get isSimpleString => components.every((x) => x is String);
     String asSimpleString() => components.join();
  +
  +  StringLiteral modify(String modifier(String input)) {
  +    return new StringLiteral.forString(modifier(asSimpleString()));
  +  }
   }
   
   class NativeCode extends Expression {
  
positive.code:   final Identifier identifier;
  
    VariableReference(this.identifier);
  
    @override
    String toString() => "VariableReference(${identifier})";
  
negative.code:   final List<Statement> statements;
  
    Block(this.statements);
    Block.forSingle(Statement statement) : this([statement]);
    Block.forSingleExpression(Expression expr) : this.forSingle(new ExpressionStatement(expr));
  
    @override
    String toString() => "Block(${statements})";
  