lib/less/tree/dimension.js
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);
}
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);
}
A number with a unit