function ensureAsync(fn) {
    if (isAsync(fn)) return fn;
    return initialParams(function (args, callback) {
        var sync = true;
        args.push(function () {
            var innerArgs = arguments;
            if (sync) {
                setImmediate$1(function () {
                    callback.apply(null, innerArgs);
                });
            } else {
                callback.apply(null, innerArgs);
            }
        });
        fn.apply(this, args);
        sync = false;
    });
}
Wrap an async function and ensure it calls its callback on a later tick of
the event loop.  If the function already calls its callback on a next tick,
no extra deferral is added. This is useful for preventing stack overflows
(`RangeError: Maximum call stack size exceeded`) and generally keeping
[Zalgo](http://blog.izs.me/post/59142742143/designing-apis-for-asynchrony)
contained. ES2017 `async` functions are returned as-is -- they are immune
to Zalgo's corrupting influences, as they always resolve on a later tick.

@name ensureAsync
@static
@memberOf module:Utils
@method
@category Util
@param {AsyncFunction} fn - an async function, one that expects a node-style
callback as its last argument.
@returns {AsyncFunction} Returns a wrapped function with the exact same call
signature as the function passed in.
@example

function sometimesAsync(arg, callback) {
    if (cache[arg]) {
        return callback(null, cache[arg]); // this would be synchronous!!
    } else {
        doSomeIO(arg, callback); // this IO would be asynchronous
    }
}

// this has a risk of stack overflows if many results are cached in a row
async.mapSeries(args, sometimesAsync, done);

// this will defer sometimesAsync's callback if necessary,
// preventing stack overflows
async.mapSeries(args, async.ensureAsync(sometimesAsync), done);