Update golang.org/x/net/internal/nettest to x/net git rev 9b4f9f5ad519
for:

   internal/nettest: add AIX operating system
   https://golang.org/cl/144077

This fixes the build failure of the vendored x/net/internal/nettest on
aix/ppc64.

Additionally this also pulls in:

  all: re-adjust build constraints for JS and NaCl
  https://golang.org/cl/122539

Updates #25893

Change-Id: I9abefc7d4ad158e9e68913362f7f1320321d6f5f
Reviewed-on: https://go-review.googlesource.com/c/151301
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>