すげえ限定的なケースをちょっと比べてみただけなので、あまりに真にうけないでください.js

1.fuga.es6.js

export default {}

2.hoge.es6.js

import hoge from "fuga.js";
function foo(bar = baz, ...qux) {
  // something
}

class AcmeClass {
  constructor() {
    // wryy
  }
  acmeMethod() {
    // uryy
  }
}

3-1.hoge.traceur.js

System.registerModule("fuga.js", [], function() {
  "use strict";
  var __moduleName = "fuga.js";
  var $__default = {};
  return {get default() {
      return $__default;
    }};
});
System.registerModule("hoge.js", [], function() {
  "use strict";
  var __moduleName = "hoge.js";
  var hoge = System.get("fuga.js").default;
  function foo() {
    var bar = arguments[0] !== (void 0) ? arguments[0] : baz;
    for (var qux = [],
        $__2 = 1; $__2 < arguments.length; $__2++)
      qux[$__2 - 1] = arguments[$__2];
  }
  var AcmeClass = function AcmeClass() {};
  ($traceurRuntime.createClass)(AcmeClass, {acmeMethod: function() {}}, {});
  return {};
});
System.get("hoge.js" + '');

3-2.hoge.6to5.js

"use strict";
var _slice = Array.prototype.slice;
var hoge = require("fuga.js").default;
function foo(bar) // something
{
  var qux = _slice.call(arguments, 1);

  if (bar === undefined)
    bar = baz;
}

var AcmeClass = function() {
  var AcmeClass = function AcmeClass() // wryy
  {
    // wryy
  };

  Object.defineProperties(AcmeClass.prototype, {
    acmeMethod: {
      writable: true,

      value: function() // uryy
      {
        // uryy
      }
    }
  });

  return AcmeClass;
}();