Creating a shebang pointing portably to an interpreter in the folder of a script

I have a JS file (file.js) that I want to have executed as a command line shell script through nodejs (iojs actually); I'm using MINGW Git Bash on Windows.

The standard way to do it is to put the following shebang in the JS file:

#!/usr/bin/env node

However, I want to pass a cmd line flag to node to activate V8 harmony features, i.e. I want an equivalent of

node --harmony_arrow_functions file.js

when run just as

./file.js

without the need to pass the flag manually.

Using

#!/usr/bin/env node --harmony_arrow_functions

does not work.

After considering http://ift.tt/1zJNfOP and http://ift.tt/1uPiJLY I created the following solution:

$ ls
file.js nodeharmony.sh

$ cat nodeharmony.sh
#!/bin/sh
exec node --harmony_arrow_functions "$@"

$ head -1 file.js
#!/bin/sh nodeharmony.sh               # or ./nodeharmony.sh, doesn't matter

$ ./file.js    # this works fine

But the problem is that when I execute it from another folder, the shell tries to find nodeharmony.sh in the current working directory, not the directory of file.js:

$ cd ..
$ ./subfolder/file.js
/bin/sh: ./nodeharmony.sh: No such file or directory

Is there a way to create a portable shebang in file.js such that I can run file.js from whatever folder, without resorting to having my custom interpreter (nodeharmony.sh) available in the PATH?