anchor.diff:    bool _isConvertable(String unit) => _conversions.containsKey(unit);
   
     // Returns [unit1]s per [unit2].
  -  int _conversionFactor(String unit1, String unit2) {
  +  num _conversionFactor(String unit1, String unit2) {
  +    if (unit1 == unit2) return 1;
       var innerMap = _conversions[unit1];
       if (innerMap == null) return null;
       return innerMap[unit2];
  
positive.code:     }
  
      var value = this.value;
      var mutableDenominators = denominatorUnits.toList();
      for (var numerator in newNumerators) {
        removeFirstWhere(mutableDenominators, (denominator) {
          var factor = _conversionFactor(numerator, denominator);
          if (factor == null) return false;
          value *= factor;
          return true;
  
negative.code: 
    Value minus(Value other) {
      if (other is SassNumber) {
        return new SassNumber(_coerceUnits(other, (num1, num2) => num1 - num2));
      }
      if (other is! SassColor) return super.minus(other);
      throw new InternalException('Undefined operation "$this - $other".');
  