TODO might need a naive tree-shaking

[3cp]

I am not a fan of tree-shaking.

But I might need to implement one, because Aurelia 2 will ship an umbrella npm package to provide a single import entry point which will ease the usage. We need tree-shaking to remove unused re-exports.

If I do implement tree-shaking, it will be a very naive one, only deal with re-exports, do not remove unused code blocks.