code: public void setLanguageOut(LanguageMode languageOut) {
      checkState(languageOut != LanguageMode.UNSUPPORTED);
      if (languageOut == LanguageMode.NO_TRANSPILE) {
        languageOutIsDefaultStrict = Optional.absent();
        outputFeatureSet = Optional.absent();
      } else {
        languageOut = languageOut == LanguageMode.STABLE ? LanguageMode.STABLE_OUT : languageOut;
        languageOutIsDefaultStrict = Optional.of(languageOut.isDefaultStrict());
        setOutputFeatureSet(languageOut.toFeatureSet());
      }
    }
docs: Sets ECMAScript version to use for the output.
  
  <p>If you are not transpiling from one version to another, use #setLanguage instead.
  
  <p>If you you need something more fine grained (e.g. "ES2017 without modules") use
  #setOutputFeatureSet.