These are snippets of code i'm using to swap out development versions of scripts to prebuilt mins that are in the same directory if available. this code is added to htmlprocessor.js in the grunt-usemin plugin ~ node_modules/grunt-usemin/lib/htmlprocessor.js

htmlprocessor.snippets.js

<!-- place at ~line 3 -->
<!-- 'use strict'; -->
<!-- var path = require('path'); -->
<!-- start include -->
var fs = require('fs');

var useMinIfAvailable = function (filePath) {
  var minFilePath = filePath.replace(/(\.js|\.css)$/g,'.min$1');

  if (fs.existsSync(minFilePath)) {
    return minFilePath;
  }
  return filePath;
};
<!-- end include -->


<!-- place at ~line 106 -->
<!--     if (block && last) { -->
<!--       var asset = l.match(/(href|src)=["']([^'"]+)["']/); -->
<!--       if (asset && asset[2]) { -->
<!-- start replace -->
        last.src.push(useMinIfAvailable(path.join(originDir, asset[2])));
<!--  end replace -->
<!--         // RequireJS uses a data-main attribute on the script tag to tell it -->
<!--         // to load up the main entry point of the amp app -->