code: function addNewerLanguageTranspilationCheck(modeName, isSupported) {
      if (transpilationRequiredForAllLaterModes) {
        requiresTranspilation[modeName] = true;
      } else if (isSupported()) {
        requiresTranspilation[modeName] = false;
      } else {
        requiresTranspilation[modeName] = true;
        transpilationRequiredForAllLaterModes = true;
      }
    }
docs: Adds an entry to requiresTranspliation for the given language mode.
  
  IMPORTANT: Calls must be made in order from oldest to newest language
  mode.
  @param {string} modeName
  @param {function(): boolean} isSupported Returns true if the JS engine
  supports the given mode.