It looks like he takes the same approach as Revel. From the comments:

    // Devweb is a simple environment for developing a web server.
    // It runs its own web server on the given address and proxies
    // all requests to the http server program named by importpath.
    // It takes care of recompiling and restarting the program as needed.

And, he uses "go build" to build the program:

    out, err := exec.Command("go", "build", "-o", "prox.exe", rootPackage).CombinedOutput()

It appears to do more than Revel does, though -- I will look at it more closely to see what extra it provides. Wish I had seen this before!