code: function writeFile(output, filePath, content) {
      var rootFolder = output.getRoot();
      filePath = path.join(rootFolder, filePath);
  
      return fs.ensureFile(filePath)
      .then(function() {
          return fs.writeFile(filePath, content);
      })
      .thenResolve(output);
  }
docs: Write a file to the output folder
  
  @param {Output} output
  @param {String} filePath
  @param {Buffer|String} content
  @return {Promise}