trace: support Go 1.6 again

App Engine is stuck 14 months in the past and doesn't have context.

Updates golang/go#20056

Change-Id: I2cb2a3a75af07728805be8c714e4725f01a17074
Reviewed-on: https://go-review.googlesource.com/41413
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

trace/trace.go

@@ -64,7 +64,6 @@ package trace // import "golang.org/x/net/trace"
 
 import (
 	"bytes"
-	"context"
 	"fmt"
 	"html/template"
 	"io"
@@ -271,18 +270,6 @@ type contextKeyT string
 
 var contextKey = contextKeyT("golang.org/x/net/trace.Trace")
 
-// 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 represents an active request.
 type Trace interface {
 	// LazyLog adds x to the event log. It will be evaluated each time the

trace/trace_go16.go

@@ -0,0 +1,21 @@
+// 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

@@ -0,0 +1,21 @@
+// 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
+}