use an interopDefault function for external CJS deps

[aearly]

Closes #419 . Also makes async's modular builds a bit smaller. :)

[Victorystick]

Good idea. To ensure that we don't get any name collisions, we should probably add _interopRequire to the list of expected global names here. That'll ensure that any declarations like var _interopRequire will be automatically renamed. We don't want to have our _interopRequire function inadvertently overridden.

[eventualbuddha]

I think this should be done in two steps like it was before, otherwise using browserify or the like on the output won't really work. Unless I'm missing something?

[eventualbuddha]

That is, there should still be a require('jquery') for such tools to find.

[eventualbuddha]

i.e.

function _interopRequire (mod) { return 'default' in mod ? mod['default'] : mod; }
var $ = _interopRequire(require('jquery'));

[aearly]

Yeah, you're right -- it'll have to be a two-step process.

[Victorystick]

I agree with @eventualbuddha.

@aearly Also just noticed that the alphabet test is broken after these changes. The current behaviour

var alphabet = require('alphabet');
var alphabet__default = 'default' in alphabet ? alphabet['default'] : alphabet;

This PR

// alphabet equals alphabet__default, although it shouldn't
var alphabet = _interopRequire('alphabet');
var alphabet__default = alphabet;

What we want is to extract the 'default' in alphabet ? alphabet['default'] : alphabet into a common function, and execute that on the whole import.

var alphabet = require('alphabet');
var alphabet__default = _interopRequire(alphabet);

[aearly]

One other thing -- we can also do something similar for the AMD/UMD/IIFE outputs too by modifying finalisers/shared/getInteropBlock.js. I wanted to get the CJS output working first.

[Rich-Harris]

awesome, thanks @aearly!