validateAlias(option, alias) {
    let key = alias.key;
    let value = alias.value;

    if (!this.optionsAliases[key]) {
      return true;
    } else {
      if (value[0] !== this.optionsAliases[key][0]) {
        throw new SilentError(
          `The "${key}" alias is already in use by the "${this.optionsAliases[key][0]}" option ` +
            `and cannot be used by the "${value[0]}" option. Please use a different alias.`
        );
      } else if (value[1] !== this.optionsAliases[key][1]) {
        this.ui.writeLine(chalk.yellow(`The "${key}" alias cannot be overridden. Please use a different alias.`));
        // delete offending alias from options
        let index = this.availableOptions.indexOf(option);
        let aliasIndex = this.availableOptions[index].aliases.indexOf(alias.original);
        if (this.availableOptions[index].aliases[aliasIndex]) {
          // first one wins
          this.availableOptions[index].aliases = [this.availableOptions[index].aliases[0]];
        }
      }
      return false;
    }
  }
Validates alias value
    @method validateAlias
    @param {Object} alias
    @return {Boolean}