code: public void setLanguageIn(LanguageMode languageIn) {
      checkState(languageIn != LanguageMode.NO_TRANSPILE);
      checkState(languageIn != LanguageMode.UNSUPPORTED);
      this.languageIn = languageIn == LanguageMode.STABLE ? LanguageMode.STABLE_IN : languageIn;
    }
docs: Sets ECMAScript version to use for the input. If you are not
  transpiling from one version to another, use #setLanguage instead.