google: remove code duplication, note appenginevm case

Before AppEngine classic allowed "google.golang.org/appengine" imports,
we used to maintain two hook files to choose either from "appengine" or
"google.golang.org/appengine" namespaces. Now, both environments allow
importing from "google.golang.org/appengine". Therefore, there
is no need to set hooks in two separate files.

Also note that Flex prefers to use metadata server, so we still
need to be able to differentiate between these environments.

Change-Id: I7478ebdfa1b062d466aaf2aca938f93d42b4c58a
Reviewed-on: https://go-review.googlesource.com/37378
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

google/appengine.go

@@ -14,8 +14,8 @@ import (
 	"golang.org/x/oauth2"
 )
 
-// Set at init time by appenginevm_hook.go. If true, we are on App Engine Managed VMs.
-var appengineVM bool
+// appengineFlex is set at init time by appengineflex_hook.go. If true, we are on App Engine Flex.
+var appengineFlex bool
 
 // Set at init time by appengine_hook.go. If nil, we're not on App Engine.
 var appengineTokenFunc func(c context.Context, scopes ...string) (token string, expiry time.Time, err error)

google/appengine_hook.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build appengine
+// +build appengine appenginevm
 
 package google
 

google/appengineflex_hook.go

@@ -0,0 +1,11 @@
+// Copyright 2015 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 appenginevm
+
+package google
+
+func init() {
+	appengineFlex = true // Flex doesn't support appengine.AccessToken; depend on metadata server.
+}

google/default.go

@@ -81,7 +81,7 @@ func FindDefaultCredentials(ctx context.Context, scope ...string) (*DefaultCrede
 	}
 
 	// Third, if we're on Google App Engine use those credentials.
-	if appengineTokenFunc != nil && !appengineVM {
+	if appengineTokenFunc != nil && !appengineFlex {
 		return &DefaultCredentials{
 			ProjectID:   appengineAppIDFunc(ctx),
 			TokenSource: AppEngineTokenSource(ctx, scope...),