Since these are type aliases in Go 1.9 make rewriting
golang.org/x/net/context imports to context the default.

Change-Id: Ib7ee58b59ea5463b92e1590c8b8f5c0ef87d310b
Reviewed-on: https://go-review.googlesource.com/58590Reviewed-by: Sameer Ajmani <sameer@golang.org>
Run-TryBot: Sam Whited <sam@samwhited.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>