This change fixes the build breakage of h2i on JS and NaCl, and avoids
using unintentional code path on JS.

Change-Id: Ib08f0f6d1d79aecc9bf1e9ee7de52b0a57c22baf
Reviewed-on: https://go-review.googlesource.com/122539
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>