The playground that you see at http://gopherjs.org/play/ is actually inside an iframe pointing to http://gopherjs.org/playground/index.html.

But the code uses the current frame's URL to generate the shared link.

So, sharing kinda works, but the generated link has the form of:

http://gopherjs.org/playground/index.html#/Myqj0bQVev

Which takes you out of the iframe with the nicer URL. And http://gopherjs.org/play/#/Myqj0bQVev does not work because it doesn't read the URL of the parent frame.