This commit fixes instantiation of the wasm module even if some of the
improted APIs don't exist. This extends the functionality initially
added in #409 to attempt to gracefully allow importing values from the
environment which don't actually exist in all contexts. In addition to
nonexistent methods being handled now entire nonexistent types are now
also handled.
I suspect that eventually we'll add a CLI flag to `wasm-bindgen` to say
"I assert everything exists, don't check it" to trim out the extra JS
glue generated here. In the meantime though this'll pave the way for a
wasm-bindgen shim to be instantiated in both a web worker and the main
thread, while using DOM-like APIs only on the main thread.
These compiler bugs have now been fixed on nightly, so we just need to
wait for the bug fixes to ride the trains to be available to everyone!
Closes#201
First added in #161 this never ended up panning out, so let's remove the
experimental suport which isn't actually used by anything today and hold off on
any other changes until an RFC happens.
