I try to build my own playground, but get the "panic: $copy is not defined" error message in output box.
Steps:

1. install gopherjs
2. install gopherjs.github.io
3. execute gopherjs.github.io/playground/update.sh
4. execute gopherjs serve to start a server
5. visit http://localhost:8080/github.com/gopherjs/gopherjs.github.io/playground/ and press Run button

But get the error message in output box.

And I found the missing function $copy is move to types.go file at gopherjs/gopherjs@a90f655

hope it is useful.