code: function(inDir, outDir, relativePath, newCache) {
      var ext = this._matchingFileExtension(relativePath);
      var moduleName = relativePath.slice(0, relativePath.length - (ext.length + 1));
      var fullInputPath = path.join(inDir, relativePath);
      var fullOutputPath = path.join(outDir, moduleName + '.' + this.targetExtension);
  
      var entry = this._transpileThroughCache(
        moduleName,
        fs.readFileSync(fullInputPath, 'utf-8'),
        newCache
      );
  
      mkdirp.sync(path.dirname(fullOutputPath));
      fs.writeFileSync(fullOutputPath, entry.output);
    }
docs: Normalizes module name, input path, and output path
  then calls transpileThroughCache to get a transpiled
  version of the ES6 source.