rate: remove Go 1.6 support, use std context

This is part of the general cleanup effort now that App Engine
requires Go 1.9+.

Related: CL 145677, CL 148277, CL 145202, CL 143717, CL 146837.

Change-Id: I831415f7484acb823cf8ac20db05a4a001df4c35
Reviewed-on: https://go-review.googlesource.com/c/148417
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

rate/rate.go

diff --git a/rate/rate.go b/rate/rate.go
index 7228d97..ae93e24 100644
--- a/rate/rate.go
+++ b/rate/rate.go
@@ -6,6 +6,7 @@
 package rate
 
 import (
+	"context"
 	"fmt"
 	"math"
 	"sync"
@@ -212,19 +213,8 @@
 	return &r
 }
 
-// contextContext is a temporary(?) copy of the context.Context type
-// to support both Go 1.6 using golang.org/x/net/context and Go 1.7+
-// with the built-in context package. If people ever stop using Go 1.6
-// we can remove this.
-type contextContext interface {
-	Deadline() (deadline time.Time, ok bool)
-	Done() <-chan struct{}
-	Err() error
-	Value(key interface{}) interface{}
-}
-
 // Wait is shorthand for WaitN(ctx, 1).
-func (lim *Limiter) wait(ctx contextContext) (err error) {
+func (lim *Limiter) Wait(ctx context.Context) (err error) {
 	return lim.WaitN(ctx, 1)
 }
 
@@ -232,7 +222,7 @@
 // It returns an error if n exceeds the Limiter's burst size, the Context is
 // canceled, or the expected wait time exceeds the Context's Deadline.
 // The burst limit is ignored if the rate limit is Inf.
-func (lim *Limiter) waitN(ctx contextContext, n int) (err error) {
+func (lim *Limiter) WaitN(ctx context.Context, n int) (err error) {
 	if n > lim.burst && lim.limit != Inf {
 		return fmt.Errorf("rate: Wait(n=%d) exceeds limiter's burst %d", n, lim.burst)
 	}

rate/rate_go16.go

diff --git a/rate/rate_go16.go b/rate/rate_go16.go
deleted file mode 100644
index 6bab185..0000000
--- a/rate/rate_go16.go
+++ /dev/null
@@ -1,21 +0,0 @@
-// Copyright 2017 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.
-
-// +build !go1.7
-
-package rate
-
-import "golang.org/x/net/context"
-
-// Wait is shorthand for WaitN(ctx, 1).
-func (lim *Limiter) Wait(ctx context.Context) (err error) {
-	return lim.waitN(ctx, 1)
-}
-
-// WaitN blocks until lim permits n events to happen.
-// It returns an error if n exceeds the Limiter's burst size, the Context is
-// canceled, or the expected wait time exceeds the Context's Deadline.
-func (lim *Limiter) WaitN(ctx context.Context, n int) (err error) {
-	return lim.waitN(ctx, n)
-}

rate/rate_go17.go

diff --git a/rate/rate_go17.go b/rate/rate_go17.go
deleted file mode 100644
index f90d85f..0000000
--- a/rate/rate_go17.go
+++ /dev/null
@@ -1,21 +0,0 @@
-// Copyright 2017 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.
-
-// +build go1.7
-
-package rate
-
-import "context"
-
-// Wait is shorthand for WaitN(ctx, 1).
-func (lim *Limiter) Wait(ctx context.Context) (err error) {
-	return lim.waitN(ctx, 1)
-}
-
-// WaitN blocks until lim permits n events to happen.
-// It returns an error if n exceeds the Limiter's burst size, the Context is
-// canceled, or the expected wait time exceeds the Context's Deadline.
-func (lim *Limiter) WaitN(ctx context.Context, n int) (err error) {
-	return lim.waitN(ctx, n)
-}