You can test it like this:
git clone https://github.com/haxefoundation/haxelib
git clone https://github.com/jgranick/haxelib-shim
cd haxelib-shim
haxe build.hxml
In order to test the update behavior, add to "Main.hx" at line 137:
Sys.println ("Upgraded haxelib client: " + upgradedClient);
To install on Windows, copy "haxelib.exe" to "C:\HaxeToolkit\haxe\haxelib.exe"
To install on Mac and Linux, copy "haxelib" to "/usr/lib/haxe/haxelib"
On Mac, I also had to replace "/usr/bin/haxelib", since it was not a symlink.
On a fresh install of Haxe, upgradedClient should be null. It uses the compiled version of haxelib.
You can use haxelib set haxelib_client to any version, it should return this value
You can also use haxelib dev path/to/haxelib to use a development version