processAddonBuildSteps(buildStep, results) {
    let addonPromises = [];
    if (this.project && this.project.addons.length) {
      addonPromises = this.project.addons.reduce((sum, addon) => {
        let method = addon[buildStep];
        if (method) {
          let val = method.call(addon, results);
          if (val) {
            sum.push(val);
          }
        }
        return sum;
      }, []);
    }

    return Promise.all(addonPromises).then(() => results);
  }
@private
@method processAddonBuildSteps
@param buildStep
@param results
@return {Promise}