Use existing helper function instead of importing "net/http". This
allows the go_bootstrap build to not depend on "net/http" package.
See cmd/go/http.go for details.

Fixes build bootstrap build with all.bash.

Change-Id: I2fd0fb01af7774f1690a353af22137680ec78170
Reviewed-on: https://go-review.googlesource.com/28531Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>