code: function (value, unit) {
      this.value = parseFloat(value);
      if (isNaN(this.value)) {
          throw new Error('Dimension is not a number.');
      }
      this.unit = (unit && unit instanceof Unit) ? unit :
        new Unit(unit ? [unit] : undefined);
      this.setParent(this.unit, this);
  }
docs: A number with a unit