I was wondering why cmd/go includes the HTTP server implementations.

Dumping the linker's deadcode dependency graph into a file and doing
some graph analysis, I found that the only reason cmd/go included an
HTTP server was because the maxBytesReader type (used by both the HTTP
transport & HTTP server) did a static type assertion to an HTTP server
type.

Changing it to a interface type assertion reduces the size of cmd/go
by 533KB (5.2%)

On linux/amd64, cmd/go goes from 10549200 to 10002624 bytes.

Add a test too so this doesn't regress. The test uses cmd/go as the
binary to test (a binary which needs the HTTP client but not the HTTP
server), but this change and test are equally applicable to any such
program.

Change-Id: I93865f43ec03b06d09241fbd9ea381817c2909c5
Reviewed-on: https://go-review.googlesource.com/20763Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>