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 <[email protected]>

Author: bradfitz

rate/rate.go

@@ -6,6 +6,7 @@
 package rate
 
 import (
+	"context"
 	"fmt"
 	"math"
 	"sync"
@@ -212,27 +213,16 @@ func (lim *Limiter) ReserveN(now time.Time, n int) *Reservation {
 	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)
 }
 
 // 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.
 // 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)
 	}