STDWEB_PRIVATE will be null if use stdweb only from other crate (0.4.0)

I just updated my project to use 0.4.0 and found that if my main crate only use stdweb from other crate, (i.e the main crate itself do not depend on stdweb, but it depend on a crate which is depends on stdweb)

when run it in browser, it will fail : 
```
Error loading Rust wasm module 'unigame': TypeError: Cannot read property 'from_js' of undefined
```
but It works in 0.3.