anchor.diff:      var string = arguments[0].assertString("string");
       var start = arguments[1].assertNumber("start-at");
       var end = arguments[2].assertNumber("end-at");
  -    start.assertNoUnits();
  -    end.assertNoUnits();
  +    start.assertNoUnits("start");
  +    end.assertNoUnits("end");
   
       var lengthInCodepoints = string.text.runes.length;
       var startCodepoint =
  
positive.code: import 'value.dart';
  
  final _microsoftFilterStart = new RegExp(r'^[a-zA-Z]+\s*=');
  
  void defineCoreFunctions(Environment environment) {
    // ## RGB
  
negative.code:   environment
        .setFunction(new BuiltInCallable("percentage", r"$number", (arguments) {
      var number = arguments[0].assertNumber("number");
      number.assertNoUnits("number");
      return new SassNumber(number.value * 100, '%');
    }));
  
  