anchor.diff:  import 'environment.dart';
   import 'exception.dart';
   import 'value.dart';
  +
  +final _microsoftFilterStart = new RegExp(r'^[a-zA-Z]+\s*=');
   
   void defineCoreFunctions(Environment environment) {
     // ## RGB
  
positive.code: 
      return _mix(color, inverse, weight);
    }));
  
    // ## Introspection
  
  
negative.code:   environment.setFunction(
        new BuiltInCallable("invert", r"$color, $weight: 50%", (arguments) {
      if (arguments[0] is SassNumber) {
        // TODO: find some way of ensuring this is stringified using the right
        // options. We may need to resort to zones.
        return new SassString("invert(${arguments[0]})");
      }
  
  