How to use the @material/select.MDCSelectFoundation.cssClasses function in @material/select

checkValidity: () => {
        const classList = this.root.ref && this.root.ref.classList;
        if (
          classList &&
          classList.contains(MDCSelectFoundation.cssClasses.REQUIRED) &&
          !classList.contains(MDCSelectFoundation.cssClasses.DISABLED)
        ) {
          // See notes for required attribute under https://www.w3.org/TR/html52/sec-forms.html#the-select-element
          // TL;DR: Invalid if no index is selected, or if the first index is selected and has an empty value.
          return (
            this.state.selectedIndex !== -1 &&
            (this.state.selectedIndex !== 0 || !!this.value)
          );
        } else {
          return true;
        }
      },
      setValid: (isValid: boolean) => {

checkValidity: () => {
        const classList = this.root.ref && this.root.ref.classList;
        if (
          classList &&
          classList.contains(MDCSelectFoundation.cssClasses.REQUIRED) &&
          !classList.contains(MDCSelectFoundation.cssClasses.DISABLED)
        ) {
          // See notes for required attribute under https://www.w3.org/TR/html52/sec-forms.html#the-select-element
          // TL;DR: Invalid if no index is selected, or if the first index is selected and has an empty value.
          return (
            this.state.selectedIndex !== -1 &&
            (this.state.selectedIndex !== 0 || !!this.value)
          );
        } else {
          return true;
        }
      },
      setValid: (isValid: boolean) => {