old_contents: var postcss = require("postcss")
  var assign = require("object-assign")
  
  module.exports = function() {
      this.filter("postcss", function (data, config) {
          // Combine our config with defaults using Object.assign
          let fullConfig = assign({}, { plugins: [], options: {} }, config)
  
          // Return our promise out so we can wrap this async promise-based API
          return new Promise((resolve, reject) => {
              // Pull in the plugins list
              postcss(fullConfig.plugins)
                  // Process the CSS with PostCSS and our options object passed to PostCSS
                  .process(data.toString(), fullConfig.options)
                  .then((result) => {
                      // Resolve our CSS the way Fly expects it
                      return resolve({ css: result.css })
                  })
                  .catch((err) => {
                      return reject(err)
                  })
          })
      })
  }
  
new_contents: var postcss = require("postcss")
  var assign = require("object-assign")
  
  module.exports = function() {
      this.filter("postcss", function (data, config) {
          // Combine our config with defaults using Object.assign
          config = assign({}, {plugins: [], options: {}}, config)
  
          // Return our promise out so we can wrap this async promise-based API
          return new Promise(function (resolve, reject) {
              // Pull in the plugins list
              postcss(config.plugins)
                  // Process the CSS with PostCSS and our options object passed to PostCSS
                  .process(data.toString(), config.options)
                  .then(function (result) {
                      // Resolve our CSS the way Fly expects it
                      return resolve({css: result.css})
                  })
                  .catch(function (err) {
                      return reject(err)
                  })
          })
      })
  }
  
subject: rewrite to es5