function reactive_import(fn) {
	var s = source(0);

	return function () {
		if (arguments.length === 1) {
			set(s, get(s) + 1);
			return arguments[0];
		} else {
			get(s);
			return fn();
		}
	};
}
Under some circumstances, imports may be reactive in legacy mode. In that case,
they should be using `reactive_import` as part of the transformation
@param {() => any} fn