old_contents: var bundleFn = arguments[3];
  var sources = arguments[4];
  var cache = arguments[5];
  
  var stringify = JSON.stringify;
  
  module.exports = function (fn) {
      var keys = [];
      var wkey;
      var cacheKeys = Object.keys(cache);
  
      for (var i = 0, l = cacheKeys.length; i < l; i++) {
          var key = cacheKeys[i];
          var exp = cache[key].exports;
          // Using babel as a transpiler to use esmodule, the export will always
          // be an object with the default export as a property of it. To ensure
          // the existing api and babel esmodule exports are both supported we
          // check for both
          if (exp === fn || exp.default === fn) {
              wkey = key;
              break;
          }
      }
  
      if (!wkey) {
          wkey = Math.floor(Math.pow(16, 8) * Math.random()).toString(16);
          var wcache = {};
          for (var i = 0, l = cacheKeys.length; i < l; i++) {
              var key = cacheKeys[i];
              wcache[key] = key;
          }
          sources[wkey] = [
              Function(['require','module','exports'], '(' + fn + ')(self)'),
              wcache
          ];
      }
      var skey = Math.floor(Math.pow(16, 8) * Math.random()).toString(16);
  
      var scache = {}; scache[wkey] = wkey;
      sources[skey] = [
          Function(['require'], (
              // try to call default if defined to also support babel esmodule
              // exports
              'var f = require(' + stringify(wkey) + ');' +
              '(f.default ? f.default : f)(self);'
          )),
          scache
      ];
  
      var src = '(' + bundleFn + ')({'
          + Object.keys(sources).map(function (key) {
              return stringify(key) + ':['
                  + sources[key][0]
                  + ',' + stringify(sources[key][1]) + ']'
              ;
          }).join(',')
          + '},{},[' + stringify(skey) + '])'
      ;
  
      var URL = window.URL || window.webkitURL || window.mozURL || window.msURL;
  
      return new Worker(URL.createObjectURL(
          new Blob([src], { type: 'text/javascript' })
      ));
  };
  
new_contents: var bundleFn = arguments[3];
  var sources = arguments[4];
  var cache = arguments[5];
  
  var stringify = JSON.stringify;
  
  module.exports = function (fn) {
      var wkey;
      var cacheKeys = Object.keys(cache);
  
      for (var i = 0, l = cacheKeys.length; i < l; i++) {
          var key = cacheKeys[i];
          var exp = cache[key].exports;
          // Using babel as a transpiler to use esmodule, the export will always
          // be an object with the default export as a property of it. To ensure
          // the existing api and babel esmodule exports are both supported we
          // check for both
          if (exp === fn || exp.default === fn) {
              wkey = key;
              break;
          }
      }
  
      if (!wkey) {
          wkey = Math.floor(Math.pow(16, 8) * Math.random()).toString(16);
          var wcache = {};
          for (var i = 0, l = cacheKeys.length; i < l; i++) {
              var key = cacheKeys[i];
              wcache[key] = key;
          }
          sources[wkey] = [
              Function(['require','module','exports'], '(' + fn + ')(self)'),
              wcache
          ];
      }
      var skey = Math.floor(Math.pow(16, 8) * Math.random()).toString(16);
  
      var scache = {}; scache[wkey] = wkey;
      sources[skey] = [
          Function(['require'], (
              // try to call default if defined to also support babel esmodule
              // exports
              'var f = require(' + stringify(wkey) + ');' +
              '(f.default ? f.default : f)(self);'
          )),
          scache
      ];
  
      var src = '(' + bundleFn + ')({'
          + Object.keys(sources).map(function (key) {
              return stringify(key) + ':['
                  + sources[key][0]
                  + ',' + stringify(sources[key][1]) + ']'
              ;
          }).join(',')
          + '},{},[' + stringify(skey) + '])'
      ;
  
      var URL = window.URL || window.webkitURL || window.mozURL || window.msURL;
  
      return new Worker(URL.createObjectURL(
          new Blob([src], { type: 'text/javascript' })
      ));
  };
  
subject: Remove keys array variable. Declared but never used.