Split bundle bugs

[jamestalmage]

Browserifies b.external(ext) method suffers from a few issues when ext is another bundle.

1. You must call b2.external(b1) before you call b1.bundle (Demonstrate bug with external bundles that are already complete browserify/browserify#1577). This makes it pretty useless for bundle splitting for karma-ava. The idea is to bundle ava once as an external to avoid the expense of bundling it for every single test.
2. It's just buggy (reproduction demonstrating #1505 browserify/browserify#1583)

lib/externalize-bundle.js pulls it's implementation directly from the browserify source. It solves 1, but 2 remains an issue.

[sindresorhus]

Have you considered using webpack?

[jamestalmage]

Yes. IMO, that is still on the table. It does allow split bundles, however, from what I can tell, the split needs to be defined up front. We can't just bundle AVA once, then add additional bundles dynamically that exclude everything in the first bundle. I haven't explored deeply enough to confirm, but I think that is the case.

Ideally AVA would have mechanisms for browser testing across a range of bundlers.