on Windows, use google-closure-compiler.cmd
on Windows, use google-closure-compiler.cmd

The `subprocess.Popen` command can't run a program without an extension
on Windows, so this change adds the extension explicitly. Adding
`shell=True` also works on Windows but seems to break building on Linux.

LLK/scratch-blocks,LLK/scratch-blocks,LLK/scratch-blocks,LLK/scratch-blocks,LLK/scratch-blocks