See https://golang.org/design/2775-binary-only-packages for design.

Fixes #2775.

Change-Id: I33e74eebffadc14d3340bba96083af0dec5172d5
Reviewed-on: https://go-review.googlesource.com/22433Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>