Lazy load for native class binding

[gfx]

Lazy loading for native class binding is necessary to use native(...) in the JSX compiler itself. That is, the web compiler try to require node modules but it always fails.

[kazuho]

IMO this should be (can be) handled by #157, although I wonder if always stripping unused code is a good idea.

OTOH, why don't you use try-catch inside the native(...) attribute to defer the problem being detected? The JavaScript expression within the native(...) attribute may return null if the load failed.