This copies the golang.org/x/net/context package to the standard library.
It is imported from the x/net repo's git rev 1d9fd3b8333e (the most
recent modified to x/net/context as of 2016-03-07).
The corresponding change to x/net/context is in https://golang.org/cl/20347
Reviewed-by: Sameer Ajmani <firstname.lastname@example.org>
Run-TryBot: Brad Fitzpatrick <email@example.com>
TryBot-Result: Gobot Gobot <firstname.lastname@example.org>
Currently only used by the client. The server is not yet wired up. A
TODO remains to document how it works server-side, once implemented.
Reviewed-by: Andrew Gerrand <email@example.com>