constructor() {
    /**
     * Mapping from a marker to a replacement.
     * @private {!Object<string, !SafeHtmlFormatter.Replacement>}
     */
    this.replacements_ = {};

    /** @private {number} Number of stored replacements. */
    this.replacementsCount_ = 0;
  }
Formatter producing SafeHtml from a plain text format and HTML fragments.
Example usage:
var formatter = new SafeHtmlFormatter();
var safeHtml = formatter.format(
    formatter.startTag('b') +
    'User input:' +
    formatter.endTag('b') +
    ' ' +
    formatter.text(userInput));
The most common usage is with goog.getMsg:
var MSG_USER_INPUT = goog.getMsg(
    '{$startLink}Learn more{$endLink} about {$userInput}', {
      'startLink': formatter.startTag('a', {'href': url}),
      'endLink': formatter.endTag('a'),
      'userInput': formatter.text(userInput)
    });
var safeHtml = formatter.format(MSG_USER_INPUT);
The formatting string should be constant with all variables processed by
formatter.text().
@final