The playground is not using GOOS=js, so it is not able to use the
package syscall/js. Examples that depend on syscall/js should not
show a "Run" button.

Fixes #28526.

Change-Id: I8b2fcdd0c0ee517a5c3864bf459f813129542389
Reviewed-on: https://go-review.googlesource.com/c/148918Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>