add library_async.js

For kripken/emscripten-fastcomp#39

They are used in fastcomp, and I will remove async-return.
On Jul 7, 2014 5:18 PM, "Alon Zakai" <notifications@github.com> wrote:

> In src/library_async.js:
>
> > +    emscripten_save_async_stack_pointer__deps: ['__async_cur_frame'],
> > +    emscripten_save_async_stack_pointer__sig: 'vi',
> > +    emscripten_save_async_stack_pointer__asm: true,
> > +    emscripten_save_async_stack_pointer: function(sp) {
> > +        sp = sp|0;
> > +        {{{ makeSetValueAsm('___async_cur_frame', 4, 'sp', 'i32') }}};
> > +    },
> > +
> > +    emscripten_check_async__deps: ['__async'],
> > +    emscripten_check_async__sig: 'i',
> > +    emscripten_check_async__asm: true,
> > +    emscripten_check_async: function() {
> > +        return ___async|0;
> > +    },
> > +
> > +    emscripten_async_return: true,
>
> why are these needed?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/kripken/emscripten/pull/2482/files#r14630848>.
>