Support CommonJS modules transpiled by Babel

[Victorystick]

The CommonJS plugin doesn't handle ES6 transpile-style exports very well. I present two examples why support must be improved:

Because this isn't very nice

// source
export default function () { return 1337; }

// after babel
"use strict";

exports.__esModule = true;

exports["default"] = function () {
  return 1337;
};

module.exports = exports["default"];

// after rollup-plugin-commonjs
var leet = (function (module) {
  var exports = module.exports;
  "use strict";

  exports.__esModule = true;

  exports["default"] = function () {
    return 1337;
  };

  module.exports = exports["default"];
  return module.exports;
})({exports:{}});

var __esModule = leet.__esModule;

export { __esModule };export default leet;

That's quite a bit of bloat.

Worse: code like this breaks

// `babel-runtime/core-js/get-iterator.js` source
module.exports = { "default": require("core-js/library/fn/get-iterator"), __esModule: true };

We do not give exports['default'] special treatment.

// after rollup-plugin-commonjs
import require$$0 from 'core-js/library/fn/get-iterator';

var getIterator = (function (module) {
var exports = module.exports;
module.exports = { "default": require$$0, __esModule: true };
return module.exports; // an object is exported, not a function
})({exports:{}});

export default getIterator;

[tdeekens]

@Victorystick sorry to bring back old memories but was this solved for you? I have trouble when the plugin seems to run over core-js code which then ends up in babel. This code seems to break my build

import '../../modules/es6.string.iterator';
import '../../modules/es6.array.from';
import '../../modules/_core';
import 'commonjs-proxy:../../modules/es6.string.iterator';
import 'commonjs-proxy:../../modules/es6.array.from';
import require$$2 from 'commonjs-proxy:../../modules/_core';

Any idea as of why or how to fix it? Help would be really greatly appreciated.