Allow compiler to accept a list of primitives that will be provided at run time

[qiemem]

It should treat invocations of those primitives as javascript calls. This gives a super easy but flexible way of doing extensions.

It should take type information with the list of primitives so that it can still do type checking.

[SethTisue]

And/or, there could be a variadic __js primitive whose first argument is a string evaluating to a JavaScript function with the remaining arguments passed to it, so you could do e.g. (__js "console.log" "hello") or whatever. All type safety goes out the window, of course.