context: implement in terms of the standard library for Go 1.7+

See https://golang.org/cl/20346

Updates golang/go#14660

Change-Id: Ia974e70cdcb240ae1df0018a07595c4d1dcd422a
Reviewed-on: https://go-review.googlesource.com/20347
Reviewed-by: Sameer Ajmani <sameer@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/context/context_test.go b/context/context_test.go
index 05345fc..fa73096 100644
--- a/context/context_test.go
+++ b/context/context_test.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build !go1.7
+
 package context
 
 import (