net: context plumbing, add Dialer.DialContext

For #12580 (http.Transport tracing/analytics)
Updates #13021

Change-Id: I126e494a7bd872e42c388ecb58499ecbf0f014cc
Reviewed-on: https://go-review.googlesource.com/22101
Run-TryBot: Brad Fitzpatrick <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Mikio Hara <[email protected]>

Author: bradfitz

src/go/build/deps_test.go

@@ -280,7 +280,9 @@ var pkgDeps = map[string][]string{
 	// Basic networking.
 	// Because net must be used by any package that wants to
 	// do networking portably, it must have a small dependency set: just L0+basic os.
-	"net": {"L0", "CGO", "math/rand", "os", "sort", "syscall", "time", "internal/syscall/windows", "internal/singleflight", "internal/race"},
+	"net": {"L0", "CGO",
+		"context", "math/rand", "os", "sort", "syscall", "time",
+		"internal/syscall/windows", "internal/singleflight", "internal/race"},
 
 	// NET enables use of basic network-related packages.
 	"NET": {

src/net/cgo_unix_test.go

@@ -7,7 +7,10 @@
 
 package net
 
-import "testing"
+import (
+	"context"
+	"testing"
+)
 
 func TestCgoLookupIP(t *testing.T) {
 	host := "localhost"
@@ -18,7 +21,7 @@ func TestCgoLookupIP(t *testing.T) {
 	if err != nil {
 		t.Error(err)
 	}
-	if _, err := goLookupIP(host); err != nil {
+	if _, err := goLookupIP(context.Background(), host); err != nil {
 		t.Error(err)
 	}
 }