code: function(inDir, outDir){
      // this object is passed through the caching process
      // and populated with newly generated or previously cached
      // values. It becomes the new cache;
      var _newTranspilerCache = {};
  
      walkSync(inDir)
        .forEach(function(relativePath) {
          if (this._shouldProcessFile(relativePath)) {
            this._handleFile(inDir, outDir, relativePath, _newTranspilerCache);
          }
        }, this);
  
      this._transpilerCache = _newTranspilerCache;
    }
docs: A hook called if ES6Modules is being used in a 1-to-1 per-per file mode
  (the default).
  
  Creates a new cache then delegates to `handleFile` for every file, which
  checks the old cache for presence of transpiled code and inserts the cached
  code (or a newly transpiled code) into the new cache.
  
  Finally, the old cache is overwritten by the new cache.