code: function(projectConfig, featureKey, variableKey, logger) {
      var feature = projectConfig.featureKeyMap[featureKey];
      if (!feature) {
        logger.log(LOG_LEVEL.ERROR, sprintf(ERROR_MESSAGES.FEATURE_NOT_IN_DATAFILE, MODULE_NAME, featureKey));
        return null;
      }
  
      var variable = feature.variableKeyMap[variableKey];
      if (!variable) {
        logger.log(LOG_LEVEL.ERROR, sprintf(ERROR_MESSAGES.VARIABLE_KEY_NOT_IN_DATAFILE, MODULE_NAME, variableKey, featureKey));
        return null;
      }
  
      return variable;
    }
docs: Get the variable with the given key associated with the feature with the
  given key. If the feature key or the variable key are invalid, log an error
  message.
  @param {Object} projectConfig
  @param {string} featureKey
  @param {string} variableKey
  @param {Object} logger
  @return {Object|null} Variable object, or null one or both of the given
  feature and variable keys are invalid