rand with stdweb feature is not compatible with --nodejs in wasm32-unkown-unknown target

[huangjj27]

after we tried fix #442
We found another incompatibility with --nodejs in wasm32-unknown-unknown:

$ cargo web test --target wasm32-unknown-unknown --nodejs --features=stdweb

   Compiling rand v0.5.0-pre.1 (file:///home/travis/build/rust-lang-nursery/rand)
    Finished release [optimized] target(s) in 2m 52.05s
    Garbage collecting "rand-d938d9acf7a83c4e.wasm"...
    Processing "rand-d938d9acf7a83c4e.wasm"...
    Finished processing of "rand-d938d9acf7a83c4e.wasm"!
/home/travis/build/rust-lang-nursery/rand/target/wasm32-unknown-unknown/release/rand-d938d9acf7a83c4e.js:38
                Module.STDWEB_PRIVATE.from_js($0, (function(){try {if (typeof window ==="object" && typeof window.crypto ==="object" && typeof window.crypto.getRandomValues ==="function"){return {success : true , ty : 1};}if (typeof require ("crypto"). randomBytes ==="function"){return {success : true , ty : 2};}return {success : false , error : new Error ("not supported")};}catch (err){return {success : false , error : err};}})());
                                      ^
TypeError: Cannot read property 'from_js' of undefined
    at __extjs_a2b16af65ac827473a87874794e6d61f99e2f317 (/home/travis/build/rust-lang-nursery/rand/target/wasm32-unknown-unknown/release/rand-d938d9acf7a83c4e.js:38:39)
    at rand::rngs::os::imp::OsRng::new::h765774a72c995801 (wasm-function[271]:97)
    at <rand::rngs::entropy::EntropyRng as rand_core::RngCore>::try_fill_bytes::h4899823cf0992da6 (wasm-function[252]:77)
    at rand::rngs::entropy::test::test_entropy::h5e484e5b8820ab29 (wasm-function[324]:41)
    at core::ops::function::FnOnce::call_once::h860ee6e300ec4137 (wasm-function[191]:13)
    at test::__rust_begin_short_backtrace::h23505ea987312149 (wasm-function[412]:3)
    at <F as alloc::boxed::FnBox<A>>::call_box::hb638f3d42164cc67 (wasm-function[411]:6)
    at std::panicking::try::do_call::h4a21a145363e9b4e (wasm-function[646]:17)
    at __rust_maybe_catch_panic (wasm-function[1501]:5)
    at test::run_test::run_test_inner::hdc7d7a13372b29ed (wasm-function[444]:801)

[pitdicker]

Testing with wasm32-unknown-unknown and nodejs is tested with every PR by Travis, ans it is supposed to work. It was just my PR which was wrong and failed.

[huangjj27]

But as the log shown above, it is in the --nodejs test that the CI build failed

[pitdicker]

I had exactly the same error locally, but it runs the tests successfully with the cargo.toml from master.