trace: remove Go 1.6 support

Change-Id: I07355ffab9e0e9437dc43cb99670736fe65bc25a
Reviewed-on: https://go-review.googlesource.com/c/148437
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

trace/trace.go

@@ -64,6 +64,7 @@ package trace // import "golang.org/x/net/trace"
 
 import (
 	"bytes"
+	"context"
 	"fmt"
 	"html/template"
 	"io"
@@ -124,6 +125,18 @@ func init() {
 	http.HandleFunc("/debug/events", Events)
 }
 
+// NewContext returns a copy of the parent context
+// and associates it with a Trace.
+func NewContext(ctx context.Context, tr Trace) context.Context {
+	return context.WithValue(ctx, contextKey, tr)
+}
+
+// FromContext returns the Trace bound to the context, if any.
+func FromContext(ctx context.Context) (tr Trace, ok bool) {
+	tr, ok = ctx.Value(contextKey).(Trace)
+	return
+}
+
 // Traces responds with traces from the program.
 // The package initialization registers it in http.DefaultServeMux
 // at /debug/requests.

trace/trace_go16.go

@@ -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 trace
-
-import "golang.org/x/net/context"
-
-// NewContext returns a copy of the parent context
-// and associates it with a Trace.
-func NewContext(ctx context.Context, tr Trace) context.Context {
-	return context.WithValue(ctx, contextKey, tr)
-}
-
-// FromContext returns the Trace bound to the context, if any.
-func FromContext(ctx context.Context) (tr Trace, ok bool) {
-	tr, ok = ctx.Value(contextKey).(Trace)
-	return
-}

trace/trace_go17.go

@@ -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 trace
-
-import "context"
-
-// NewContext returns a copy of the parent context
-// and associates it with a Trace.
-func NewContext(ctx context.Context, tr Trace) context.Context {
-	return context.WithValue(ctx, contextKey, tr)
-}
-
-// FromContext returns the Trace bound to the context, if any.
-func FromContext(ctx context.Context) (tr Trace, ok bool) {
-	tr, ok = ctx.Value(contextKey).(Trace)
-	return
-}