parseArrowFormalParameters_(asyncToken) {
    if (asyncToken)
      this.eat_(IDENTIFIER);
    this.eat_(OPEN_PAREN);
    let parameters = this.parseFormalParameters_();
    this.eat_(CLOSE_PAREN);
    return parameters;
  }
ArrowFormalParameters[Yield, GeneratorParameter] :
  ( StrictFormalParameters[?Yield, ?GeneratorParameter] )