constructor(isStrict, reporter) {
    super();
    this.reporter_ = reporter;
    this.names_ = new StringSet();
    this.errors_ = [];
    this.reportStrictKeywords_ = isStrict;
    // In the case of sloppy functions this changes when we see a default,
    // rest or destructuring param.
    this.reportDuplicates_ = isStrict;
  }
@param {boolean} isStrict
@param {ErrorReporter} reporter