google: Re-enable AppEngineTokenSource to be used from Managed VMs.

Fixes #152.

Change-Id: I757c011d3ac5dca8f80fb2119eda3adf8c178ca6
Reviewed-on: https://go-review.googlesource.com/14622
Reviewed-by: Andrew Gerrand <adg@golang.org>

google/appenginevm_hook.go

@@ -0,0 +1,14 @@
+// Copyright 2015 The oauth2 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
+
+import "google.golang.org/appengine"
+
+func init() {
+	appengineVM = true
+	appengineTokenFunc = appengine.AccessToken
+}