os.Args is usually never empty and the flag package panics if it is.
This commit makes os.Args default to ["js"] for js/wasm.

Change-Id: Iba527145686487b052da438fca40159e57e61a81
Reviewed-on: https://go-review.googlesource.com/117475Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>