code: function mergeParams(target, source) {
      Object.keys(source).forEach(function (key) {
          if (!(key in target)) {
              target[key] = [];
          }
          target[key] = target[key].concat(source[key]);
      });
  }
docs: Merges params from `source` into `target` (mutating `target`).
  
  @param {!Object<string, !Array<string>>} target
  @param {!Object<string, !Array<string>>} source