Well to be fair Javascript is kind of limited so Ryan does not have language support for coroutines unless he wants to redesign Javascript. Even better for node.js would be something a bit more CSP-like such as goroutines and channels but again - Javascript. If you want Javascript with coroutines you can always use Lua.
Speaking of Lua, it doesn't actually have any language constructs for coroutines - coroutine creation and switching is handled entirely via library functions in the `coroutine` package.
So while adding coroutine support to V8 would probably require some substantial internal rewrites, it wouldn't necessitate changing the language - just add a global Coroutine object with the coroutine-switching functions like create, resume, yield...
Right, it just needs a sufficiently powerful C API and internals designed with coroutines in mind. ("just" sounds out of place, there...)
If you want to look at the Lua coroutine implementation, start at auxresume (http://www.lua.org/source/5.1/lbaselib.c.html#auxresume) in lbaselib.c, and the functions tagged luaB_ more generally. (In 5.2, they've been moved to their own file, lcorolib.c.)
