handler = function(e) {
    if (e) {
      resolver.reject(e);
    } else {
      resolver.resolve();
    }
  }
Helper to set a value.
@param {!Firebase} ref
@param {?Object} value
@param {boolean=} opt_overwrite If true, overwrite the whole key with value;
    otherwise, update the key with value given.
@return {!IThenable}