oauth2: remove oauth2.Context type, simplify App Engine token code

You can now use the "google.golang.org/appengine" packages on both
Managed VMs and App Engine Classic(TM). The newer packages use the
context.Context instead of appengine.Context, so we no longer need the
oauth2.Context type.

Some clients will require code changes, replacing oauth2.Context or
appengine.Context with context.Context (imported from
the repository "golang.org/x/net/context").

Users of classic App Engine must switch to using the new
"google.golang.org/appengine" packages in order to use the oauth2
package.

Fixes #89

Change-Id: Ibaff3117117f9f7c5d1b3048a6e4086f62c18c3b
Reviewed-on: https://go-review.googlesource.com/6075
Reviewed-by: Burcu Dogan <jbd@google.com>

client_appengine.go

@@ -2,38 +2,23 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build appengine,!appenginevm
+// +build appengine appenginevm
 
 // App Engine hooks.
 
 package oauth2
 
 import (
-	"log"
 	"net/http"
-	"sync"
 
-	"appengine"
-	"appengine/urlfetch"
+	"golang.org/x/net/context"
+	"google.golang.org/appengine/urlfetch"
 )
 
-var warnOnce sync.Once
-
 func init() {
 	registerContextClientFunc(contextClientAppEngine)
 }
 
-func contextClientAppEngine(ctx Context) (*http.Client, error) {
-	if actx, ok := ctx.(appengine.Context); ok {
-		return urlfetch.Client(actx), nil
-	}
-	// The user did it wrong. We'll log once (and hope they see it
-	// in dev_appserver), but stil return (nil, nil) in case some
-	// other contextClientFunc hook finds a way to proceed.
-	warnOnce.Do(gaeDoingItWrongHelp)
-	return nil, nil
-}
-
-func gaeDoingItWrongHelp() {
-	log.Printf("WARNING: you attempted to use the oauth2 package without passing a valid appengine.Context or *http.Request as the oauth2.Context. App Engine requires that all service RPCs (including urlfetch) be associated with an *http.Request/appengine.Context.")
+func contextClientAppEngine(ctx context.Context) (*http.Client, error) {
+	return urlfetch.Client(ctx), nil
 }