Split static and dynamic www feature flags

[gaearon]

This makes it so that www feature flags only include things that are actually behind a GK.

In theory this would reduce the bundle size of FB bundles. In practice, it currently doesn't because even though we know more of them statically, we still end up with code like

var flag = false;

if (flag) {
  // ...
}

in production bundles. This is because we use Uglify for FB builds which is not as smart as GCC.

I have, however, verified that if we were to use GCC, it would strip them out (whereas it can’t possibly strip them out in the current version). So this seems incrementally better.

It might also be the case that the www minifer already handles conditions like this. In that case, even though it doesn’t show up as a win here, it might actually be in www after compilation.