context: add the context package from golang.org/x/net/context

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

Updates #14660

Change-Id: Ida14b1b7e115194d6218d9ac614548b9f41641cc
Reviewed-on: https://go-review.googlesource.com/20346Reviewed-by: Sameer Ajmani <sameer@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/context/withtimeout_test.go

+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package context_test
+
+import (
+	"context"
+	"fmt"
+	"time"
+)
+
+func ExampleWithTimeout() {
+	// Pass a context with a timeout to tell a blocking function that it
+	// should abandon its work after the timeout elapses.
+	ctx, _ := context.WithTimeout(context.Background(), 100*time.Millisecond)
+	select {
+	case <-time.After(200 * time.Millisecond):
+		fmt.Println("overslept")
+	case <-ctx.Done():
+		fmt.Println(ctx.Err()) // prints "context deadline exceeded"
+	}
+	// Output:
+	// context deadline exceeded
+}

src/go/build/deps_test.go

@@ -215,6 +215,7 @@ var pkgDeps = map[string][]string{
 	"compress/gzip":            {"L4", "compress/flate"},
 	"compress/lzw":             {"L4"},
 	"compress/zlib":            {"L4", "compress/flate"},
+	"context":                  {"errors", "fmt", "sync", "time"},
 	"database/sql":             {"L4", "container/list", "database/sql/driver"},
 	"database/sql/driver":      {"L4", "time"},
 	"debug/dwarf":              {"L4"},