readyStateChangeHandler_(event) {
    const xhr = /** @type {!XhrIo} */ (event.target);
    try {
      if (xhr == this.xhr_) {
        this.onReadyStateChanged_();
      } else {
        googLog.warning(this.logger_, 'Called back with an unexpected xhr.');
      }
    } catch (ex) {
      googLog.error(
          this.logger_,
          'readyStateChangeHandler_ thrown exception' +
              ' ' + ex);
      // no rethrow
      this.updateStatus_(XhrStreamReaderStatus.HANDLER_EXCEPTION);
      this.clear_();
    }
  }
Handles XHR readystatechange events.

TODO(user): throttling may be needed.

@param {!Event} event The event.
@private